feat: add timeline API with schedule-aware enrollment aggregation

Implements POST /api/timeline endpoint that aggregates enrollment by
subject over 15-minute slots, filtering courses by their actual meeting
times. Includes ISR-style schedule cache with hourly background refresh
using stale-while-revalidate pattern, database indexes for efficient
queries, and frontend refactor to dynamically discover subjects from API.

migrations/20260131200000_timeline_indexes.sql

@@ -0,0 +1,13 @@
+-- Indexes for the timeline aggregation endpoint.
+-- The query buckets course_metrics by 15-minute intervals, joins to courses
+-- for subject, and aggregates enrollment. These indexes support efficient
+-- time-range scans and the join.
+
+-- Primary access pattern: scan course_metrics by timestamp range
+CREATE INDEX IF NOT EXISTS idx_course_metrics_timestamp
+    ON course_metrics (timestamp);
+
+-- Composite index for the DISTINCT ON (bucket, course_id) ordered by timestamp DESC
+-- to efficiently pick the latest metric per course per bucket.
+CREATE INDEX IF NOT EXISTS idx_course_metrics_course_timestamp
+    ON course_metrics (course_id, timestamp DESC);

src/app.rs

@@ -85,6 +85,11 @@ impl App {
             info!(error = ?e, "Could not load reference cache on startup (may be empty)");
         }
 
+        // Load schedule cache for timeline enrollment queries
+        if let Err(e) = app_state.schedule_cache.load().await {
+            info!(error = ?e, "Could not load schedule cache on startup (may be empty)");
+        }
+
         // Seed the initial admin user if configured
         if let Some(admin_id) = config.admin_discord_id {
             let user = crate::data::users::ensure_seed_admin(&db_pool, admin_id as i64)

src/state.rs

@@ -4,6 +4,7 @@ use crate::banner::BannerApi;
 use crate::banner::Course;
 use crate::data::models::ReferenceData;
 use crate::status::ServiceStatusRegistry;
+use crate::web::schedule_cache::ScheduleCache;
 use crate::web::session_cache::{OAuthStateStore, SessionCache};
 use crate::web::ws::ScrapeJobEvent;
 use anyhow::Result;
@@ -76,12 +77,14 @@ pub struct AppState {
     pub reference_cache: Arc<RwLock<ReferenceCache>>,
     pub session_cache: SessionCache,
     pub oauth_state_store: OAuthStateStore,
+    pub schedule_cache: ScheduleCache,
     pub scrape_job_tx: broadcast::Sender<ScrapeJobEvent>,
 }
 
 impl AppState {
     pub fn new(banner_api: Arc<BannerApi>, db_pool: PgPool) -> Self {
         let (scrape_job_tx, _) = broadcast::channel(64);
+        let schedule_cache = ScheduleCache::new(db_pool.clone());
         Self {
             session_cache: SessionCache::new(db_pool.clone()),
             oauth_state_store: OAuthStateStore::new(),
@@ -89,6 +92,7 @@ impl AppState {
             db_pool,
             service_statuses: ServiceStatusRegistry::new(),
             reference_cache: Arc::new(RwLock::new(ReferenceCache::new())),
+            schedule_cache,
             scrape_job_tx,
         }
     }

src/web/mod.rs

@@ -11,7 +11,9 @@ pub mod calendar;
 pub mod encoding;
 pub mod extractors;
 pub mod routes;
+pub mod schedule_cache;
 pub mod session_cache;
+pub mod timeline;
 pub mod ws;
 
 pub use routes::*;

src/web/routes.rs

@@ -14,6 +14,7 @@ use crate::web::admin_rmp;
 use crate::web::admin_scraper;
 use crate::web::auth::{self, AuthConfig};
 use crate::web::calendar;
+use crate::web::timeline;
 use crate::web::ws;
 #[cfg(feature = "embed-assets")]
 use axum::{
@@ -54,6 +55,7 @@ pub fn create_router(app_state: AppState, auth_config: AuthConfig) -> Router {
         .route("/terms", get(get_terms))
         .route("/subjects", get(get_subjects))
         .route("/reference/{category}", get(get_reference))
+        .route("/timeline", post(timeline::timeline))
         .with_state(app_state.clone());
 
     let auth_router = Router::new()

src/web/schedule_cache.rs

@@ -0,0 +1,444 @@
+//! ISR-style schedule cache for timeline enrollment queries.
+//!
+//! Loads all courses with their meeting times from the database, parses the
+//! JSONB meeting times into a compact in-memory representation, and caches
+//! the result. The cache is refreshed in the background every hour using a
+//! stale-while-revalidate pattern with singleflight deduplication — readers
+//! always get the current cached value instantly, never blocking on a refresh.
+
+use chrono::NaiveDate;
+use serde_json::Value;
+use sqlx::PgPool;
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::Arc;
+use tokio::sync::watch;
+use tracing::{debug, error, info};
+
+/// How often the cache is considered fresh (1 hour).
+const REFRESH_INTERVAL: std::time::Duration = std::time::Duration::from_secs(60 * 60);
+
+// ── Compact schedule representation ─────────────────────────────────
+
+/// A single meeting time block, pre-parsed for fast filtering.
+#[derive(Debug, Clone)]
+pub(crate) struct ParsedSchedule {
+    /// Bitmask of days: bit 0 = Monday, bit 6 = Sunday.
+    days: u8,
+    /// Minutes since midnight for start (e.g. 600 = 10:00).
+    begin_minutes: u16,
+    /// Minutes since midnight for end (e.g. 650 = 10:50).
+    end_minutes: u16,
+    /// First day the meeting pattern is active.
+    start_date: NaiveDate,
+    /// Last day the meeting pattern is active.
+    end_date: NaiveDate,
+}
+
+/// A course with its enrollment and pre-parsed schedule blocks.
+#[derive(Debug, Clone)]
+pub(crate) struct CachedCourse {
+    pub(crate) subject: String,
+    pub(crate) enrollment: i32,
+    pub(crate) schedules: Vec<ParsedSchedule>,
+}
+
+/// The immutable snapshot of all courses, swapped atomically on refresh.
+#[derive(Debug, Clone)]
+pub(crate) struct ScheduleSnapshot {
+    pub(crate) courses: Vec<CachedCourse>,
+    refreshed_at: std::time::Instant,
+}
+
+// ── Cache handle ────────────────────────────────────────────────────
+
+/// Shared schedule cache. Clone-cheap (all `Arc`-wrapped internals).
+#[derive(Clone)]
+pub struct ScheduleCache {
+    /// Current snapshot, updated via `watch` channel for lock-free reads.
+    rx: watch::Receiver<Arc<ScheduleSnapshot>>,
+    /// Sender side, held to push new snapshots.
+    tx: Arc<watch::Sender<Arc<ScheduleSnapshot>>>,
+    /// Singleflight guard — true while a refresh task is in flight.
+    refreshing: Arc<AtomicBool>,
+    /// Database pool for refresh queries.
+    pool: PgPool,
+}
+
+impl ScheduleCache {
+    /// Create a new cache with an empty initial snapshot.
+    pub(crate) fn new(pool: PgPool) -> Self {
+        let empty = Arc::new(ScheduleSnapshot {
+            courses: Vec::new(),
+            refreshed_at: std::time::Instant::now(),
+        });
+        let (tx, rx) = watch::channel(empty);
+        Self {
+            rx,
+            tx: Arc::new(tx),
+            refreshing: Arc::new(AtomicBool::new(false)),
+            pool,
+        }
+    }
+
+    /// Get the current snapshot. Never blocks on refresh.
+    pub(crate) fn snapshot(&self) -> Arc<ScheduleSnapshot> {
+        self.rx.borrow().clone()
+    }
+
+    /// Check freshness and trigger a background refresh if stale.
+    /// Always returns immediately — the caller uses the current snapshot.
+    pub(crate) fn ensure_fresh(&self) {
+        let snap = self.rx.borrow();
+        if snap.refreshed_at.elapsed() < REFRESH_INTERVAL {
+            return;
+        }
+        // Singleflight: only one refresh at a time.
+        if self
+            .refreshing
+            .compare_exchange(false, true, Ordering::AcqRel, Ordering::Acquire)
+            .is_err()
+        {
+            debug!("Schedule cache refresh already in flight, skipping");
+            return;
+        }
+        let cache = self.clone();
+        tokio::spawn(async move {
+            match load_snapshot(&cache.pool).await {
+                Ok(snap) => {
+                    let count = snap.courses.len();
+                    let _ = cache.tx.send(Arc::new(snap));
+                    info!(courses = count, "Schedule cache refreshed");
+                }
+                Err(e) => {
+                    error!(error = %e, "Failed to refresh schedule cache");
+                }
+            }
+            cache.refreshing.store(false, Ordering::Release);
+        });
+    }
+
+    /// Force an initial load (blocking). Call once at startup.
+    pub(crate) async fn load(&self) -> anyhow::Result<()> {
+        let snap = load_snapshot(&self.pool).await?;
+        let count = snap.courses.len();
+        let _ = self.tx.send(Arc::new(snap));
+        info!(courses = count, "Schedule cache initially loaded");
+        Ok(())
+    }
+}
+
+// ── Database loading ────────────────────────────────────────────────
+
+/// Row returned from the lightweight schedule query.
+#[derive(sqlx::FromRow)]
+struct ScheduleRow {
+    subject: String,
+    enrollment: i32,
+    meeting_times: Value,
+}
+
+/// Load all courses and parse their meeting times into a snapshot.
+async fn load_snapshot(pool: &PgPool) -> anyhow::Result<ScheduleSnapshot> {
+    let start = std::time::Instant::now();
+
+    let rows: Vec<ScheduleRow> = sqlx::query_as(
+        "SELECT subject, enrollment, meeting_times FROM courses",
+    )
+    .fetch_all(pool)
+    .await?;
+
+    let courses: Vec<CachedCourse> = rows
+        .into_iter()
+        .map(|row| {
+            let schedules = parse_meeting_times(&row.meeting_times);
+            CachedCourse {
+                subject: row.subject,
+                enrollment: row.enrollment,
+                schedules,
+            }
+        })
+        .collect();
+
+    debug!(
+        courses = courses.len(),
+        elapsed_ms = start.elapsed().as_millis(),
+        "Schedule snapshot built"
+    );
+
+    Ok(ScheduleSnapshot {
+        courses,
+        refreshed_at: std::time::Instant::now(),
+    })
+}
+
+// ── Meeting time parsing ────────────────────────────────────────────
+
+/// Parse the JSONB `meeting_times` array into compact `ParsedSchedule` values.
+fn parse_meeting_times(value: &Value) -> Vec<ParsedSchedule> {
+    let Value::Array(arr) = value else {
+        return Vec::new();
+    };
+
+    arr.iter().filter_map(parse_one_meeting).collect()
+}
+
+fn parse_one_meeting(mt: &Value) -> Option<ParsedSchedule> {
+    let begin_time = mt.get("begin_time")?.as_str()?;
+    let end_time = mt.get("end_time")?.as_str()?;
+
+    let begin_minutes = parse_hhmm(begin_time)?;
+    let end_minutes = parse_hhmm(end_time)?;
+
+    if end_minutes <= begin_minutes {
+        return None;
+    }
+
+    let start_date = parse_date(mt.get("start_date")?.as_str()?)?;
+    let end_date = parse_date(mt.get("end_date")?.as_str()?)?;
+
+    const DAY_KEYS: [&str; 7] = [
+        "monday",
+        "tuesday",
+        "wednesday",
+        "thursday",
+        "friday",
+        "saturday",
+        "sunday",
+    ];
+    let mut days: u8 = 0;
+    for (bit, key) in DAY_KEYS.iter().enumerate() {
+        if mt.get(*key).and_then(Value::as_bool).unwrap_or(false) {
+            days |= 1 << bit;
+        }
+    }
+
+    // Skip meetings with no days (online async, etc.)
+    if days == 0 {
+        return None;
+    }
+
+    Some(ParsedSchedule {
+        days,
+        begin_minutes,
+        end_minutes,
+        start_date,
+        end_date,
+    })
+}
+
+/// Parse "HHMM" → minutes since midnight.
+fn parse_hhmm(s: &str) -> Option<u16> {
+    if s.len() != 4 {
+        return None;
+    }
+    let hours: u16 = s[..2].parse().ok()?;
+    let mins: u16 = s[2..].parse().ok()?;
+    if hours >= 24 || mins >= 60 {
+        return None;
+    }
+    Some(hours * 60 + mins)
+}
+
+/// Parse "MM/DD/YYYY" → NaiveDate.
+fn parse_date(s: &str) -> Option<NaiveDate> {
+    NaiveDate::parse_from_str(s, "%m/%d/%Y").ok()
+}
+
+// ── Slot matching ───────────────────────────────────────────────────
+
+/// Day-of-week as our bitmask index (Monday = 0 .. Sunday = 6).
+/// Chrono's `weekday().num_days_from_monday()` already gives 0=Mon..6=Sun.
+pub(crate) fn weekday_bit(day: chrono::Weekday) -> u8 {
+    1 << day.num_days_from_monday()
+}
+
+impl ParsedSchedule {
+    /// Check if this schedule is active during a given slot.
+    ///
+    /// `slot_date` is the calendar date of the slot.
+    /// `slot_start` / `slot_end` are minutes since midnight for the 15-min window.
+    #[inline]
+    pub(crate) fn active_during(
+        &self,
+        slot_date: NaiveDate,
+        slot_weekday_bit: u8,
+        slot_start_minutes: u16,
+        slot_end_minutes: u16,
+    ) -> bool {
+        // Day-of-week check
+        if self.days & slot_weekday_bit == 0 {
+            return false;
+        }
+        // Date range check
+        if slot_date < self.start_date || slot_date > self.end_date {
+            return false;
+        }
+        // Time overlap: meeting [begin, end) overlaps slot [start, end)
+        self.begin_minutes < slot_end_minutes && self.end_minutes > slot_start_minutes
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use chrono::NaiveDate;
+    use serde_json::json;
+
+    #[test]
+    fn parse_hhmm_valid() {
+        assert_eq!(parse_hhmm("0000"), Some(0));
+        assert_eq!(parse_hhmm("0930"), Some(570));
+        assert_eq!(parse_hhmm("1350"), Some(830));
+        assert_eq!(parse_hhmm("2359"), Some(1439));
+    }
+
+    #[test]
+    fn parse_hhmm_invalid() {
+        assert_eq!(parse_hhmm(""), None);
+        assert_eq!(parse_hhmm("abc"), None);
+        assert_eq!(parse_hhmm("2500"), None);
+        assert_eq!(parse_hhmm("0060"), None);
+    }
+
+    #[test]
+    fn parse_date_valid() {
+        assert_eq!(
+            parse_date("08/26/2025"),
+            Some(NaiveDate::from_ymd_opt(2025, 8, 26).unwrap())
+        );
+    }
+
+    #[test]
+    fn parse_meeting_times_basic() {
+        let json = json!([{
+            "begin_time": "1000",
+            "end_time": "1050",
+            "start_date": "08/26/2025",
+            "end_date": "12/13/2025",
+            "monday": true,
+            "tuesday": false,
+            "wednesday": true,
+            "thursday": false,
+            "friday": true,
+            "saturday": false,
+            "sunday": false,
+            "building": "NPB",
+            "building_description": "North Paseo Building",
+            "room": "1.238",
+            "campus": "11",
+            "meeting_type": "FF",
+            "meeting_schedule_type": "AFF"
+        }]);
+
+        let schedules = parse_meeting_times(&json);
+        assert_eq!(schedules.len(), 1);
+
+        let s = &schedules[0];
+        assert_eq!(s.begin_minutes, 600); // 10:00
+        assert_eq!(s.end_minutes, 650); // 10:50
+        assert_eq!(s.days, 0b0010101); // Mon, Wed, Fri
+    }
+
+    #[test]
+    fn parse_meeting_times_skips_null_times() {
+        let json = json!([{
+            "begin_time": null,
+            "end_time": null,
+            "start_date": "08/26/2025",
+            "end_date": "12/13/2025",
+            "monday": false,
+            "tuesday": false,
+            "wednesday": false,
+            "thursday": false,
+            "friday": false,
+            "saturday": false,
+            "sunday": false,
+            "meeting_type": "OS",
+            "meeting_schedule_type": "AFF"
+        }]);
+
+        let schedules = parse_meeting_times(&json);
+        assert!(schedules.is_empty());
+    }
+
+    #[test]
+    fn active_during_matching_slot() {
+        let sched = ParsedSchedule {
+            days: 0b0000001, // Monday
+            begin_minutes: 600,
+            end_minutes: 650,
+            start_date: NaiveDate::from_ymd_opt(2025, 8, 26).unwrap(),
+            end_date: NaiveDate::from_ymd_opt(2025, 12, 13).unwrap(),
+        };
+
+        // Monday Sept 1 2025, 10:00-10:15 slot
+        let date = NaiveDate::from_ymd_opt(2025, 9, 1).unwrap();
+        assert!(sched.active_during(date, weekday_bit(chrono::Weekday::Mon), 600, 615));
+    }
+
+    #[test]
+    fn active_during_wrong_day() {
+        let sched = ParsedSchedule {
+            days: 0b0000001, // Monday only
+            begin_minutes: 600,
+            end_minutes: 650,
+            start_date: NaiveDate::from_ymd_opt(2025, 8, 26).unwrap(),
+            end_date: NaiveDate::from_ymd_opt(2025, 12, 13).unwrap(),
+        };
+
+        // Tuesday Sept 2 2025
+        let date = NaiveDate::from_ymd_opt(2025, 9, 2).unwrap();
+        assert!(!sched.active_during(date, weekday_bit(chrono::Weekday::Tue), 600, 615));
+    }
+
+    #[test]
+    fn active_during_no_time_overlap() {
+        let sched = ParsedSchedule {
+            days: 0b0000001,
+            begin_minutes: 600, // 10:00
+            end_minutes: 650,   // 10:50
+            start_date: NaiveDate::from_ymd_opt(2025, 8, 26).unwrap(),
+            end_date: NaiveDate::from_ymd_opt(2025, 12, 13).unwrap(),
+        };
+
+        let date = NaiveDate::from_ymd_opt(2025, 9, 1).unwrap(); // Monday
+        // Slot 11:00-11:15 — after the meeting ends
+        assert!(!sched.active_during(date, weekday_bit(chrono::Weekday::Mon), 660, 675));
+        // Slot 9:45-10:00 — just before meeting starts (end=600, begin=600 → no overlap)
+        assert!(!sched.active_during(date, weekday_bit(chrono::Weekday::Mon), 585, 600));
+    }
+
+    #[test]
+    fn active_during_outside_date_range() {
+        let sched = ParsedSchedule {
+            days: 0b0000001,
+            begin_minutes: 600,
+            end_minutes: 650,
+            start_date: NaiveDate::from_ymd_opt(2025, 8, 26).unwrap(),
+            end_date: NaiveDate::from_ymd_opt(2025, 12, 13).unwrap(),
+        };
+
+        // Monday Jan 6 2025 — before semester
+        let date = NaiveDate::from_ymd_opt(2025, 1, 6).unwrap();
+        assert!(!sched.active_during(date, weekday_bit(chrono::Weekday::Mon), 600, 615));
+    }
+
+    #[test]
+    fn active_during_edge_overlap() {
+        let sched = ParsedSchedule {
+            days: 0b0000001,
+            begin_minutes: 600,
+            end_minutes: 650,
+            start_date: NaiveDate::from_ymd_opt(2025, 8, 26).unwrap(),
+            end_date: NaiveDate::from_ymd_opt(2025, 12, 13).unwrap(),
+        };
+
+        let date = NaiveDate::from_ymd_opt(2025, 9, 1).unwrap();
+        // Slot 10:45-11:00 — overlaps last 5 minutes of meeting
+        assert!(sched.active_during(date, weekday_bit(chrono::Weekday::Mon), 645, 660));
+        // Slot 9:45-10:00 — ends exactly when meeting starts, no overlap
+        assert!(!sched.active_during(date, weekday_bit(chrono::Weekday::Mon), 585, 600));
+        // Slot 10:50-11:05 — starts exactly when meeting ends, no overlap
+        assert!(!sched.active_during(date, weekday_bit(chrono::Weekday::Mon), 650, 665));
+    }
+}

src/web/timeline.rs

@@ -0,0 +1,259 @@
+//! Timeline API endpoint for enrollment aggregation by subject over time.
+//!
+//! Accepts multiple time ranges, merges overlaps, aligns to 15-minute
+//! slot boundaries, and returns per-subject enrollment totals for each slot.
+//! Only courses whose meeting times overlap a given slot contribute to that
+//! slot's totals — so the chart reflects the actual class schedule rhythm.
+//!
+//! Course data is served from an ISR-style in-memory cache (see
+//! [`ScheduleCache`]) that refreshes hourly in the background with
+//! stale-while-revalidate semantics.
+
+use axum::{
+    extract::State,
+    http::StatusCode,
+    response::{IntoResponse, Json, Response},
+};
+use chrono::{DateTime, Datelike, Duration, NaiveTime, Timelike, Utc};
+use chrono_tz::US::Central;
+use serde::{Deserialize, Serialize};
+use std::collections::{BTreeMap, BTreeSet};
+use ts_rs::TS;
+
+use crate::state::AppState;
+use crate::web::schedule_cache::weekday_bit;
+
+/// 15 minutes in seconds, matching the frontend `SLOT_INTERVAL_MS`.
+const SLOT_SECONDS: i64 = 15 * 60;
+const SLOT_MINUTES: u16 = 15;
+
+/// Maximum number of ranges in a single request.
+const MAX_RANGES: usize = 20;
+
+/// Maximum span of a single range (72 hours).
+const MAX_RANGE_SPAN: Duration = Duration::hours(72);
+
+/// Maximum total span across all ranges to prevent excessive queries.
+const MAX_TOTAL_SPAN: Duration = Duration::hours(168); // 1 week
+
+// ── Request / Response types ────────────────────────────────────────
+
+#[derive(Debug, Deserialize)]
+pub(crate) struct TimelineRequest {
+    ranges: Vec<TimeRange>,
+}
+
+#[derive(Debug, Deserialize)]
+pub(crate) struct TimeRange {
+    start: DateTime<Utc>,
+    end: DateTime<Utc>,
+}
+
+#[derive(Debug, Serialize, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export)]
+pub struct TimelineResponse {
+    /// 15-minute slots with per-subject enrollment totals, sorted by time.
+    slots: Vec<TimelineSlot>,
+    /// All subject codes present in the returned data.
+    subjects: Vec<String>,
+}
+
+#[derive(Debug, Serialize, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export)]
+pub struct TimelineSlot {
+    /// ISO-8601 timestamp at the start of this 15-minute bucket.
+    time: DateTime<Utc>,
+    /// Subject code → total enrollment in this slot.
+    subjects: BTreeMap<String, i64>,
+}
+
+// ── Error type ──────────────────────────────────────────────────────
+
+pub(crate) struct TimelineError {
+    status: StatusCode,
+    message: String,
+}
+
+impl TimelineError {
+    fn bad_request(msg: impl Into<String>) -> Self {
+        Self {
+            status: StatusCode::BAD_REQUEST,
+            message: msg.into(),
+        }
+    }
+}
+
+impl IntoResponse for TimelineError {
+    fn into_response(self) -> Response {
+        (
+            self.status,
+            Json(serde_json::json!({ "error": self.message })),
+        )
+            .into_response()
+    }
+}
+
+// ── Alignment helpers ───────────────────────────────────────────────
+
+/// Floor a timestamp to the nearest 15-minute boundary.
+fn align_floor(ts: DateTime<Utc>) -> DateTime<Utc> {
+    let secs = ts.timestamp();
+    let aligned = (secs / SLOT_SECONDS) * SLOT_SECONDS;
+    DateTime::from_timestamp(aligned, 0).unwrap_or(ts)
+}
+
+/// Ceil a timestamp to the nearest 15-minute boundary.
+fn align_ceil(ts: DateTime<Utc>) -> DateTime<Utc> {
+    let secs = ts.timestamp();
+    let aligned = ((secs + SLOT_SECONDS - 1) / SLOT_SECONDS) * SLOT_SECONDS;
+    DateTime::from_timestamp(aligned, 0).unwrap_or(ts)
+}
+
+// ── Range merging ───────────────────────────────────────────────────
+
+/// Aligned, validated range.
+#[derive(Debug, Clone, Copy)]
+struct AlignedRange {
+    start: DateTime<Utc>,
+    end: DateTime<Utc>,
+}
+
+/// Merge overlapping/adjacent ranges into a minimal set.
+fn merge_ranges(mut ranges: Vec<AlignedRange>) -> Vec<AlignedRange> {
+    if ranges.is_empty() {
+        return ranges;
+    }
+    ranges.sort_by_key(|r| r.start);
+    let mut merged: Vec<AlignedRange> = vec![ranges[0]];
+    for r in &ranges[1..] {
+        let last = merged.last_mut().unwrap();
+        if r.start <= last.end {
+            last.end = last.end.max(r.end);
+        } else {
+            merged.push(*r);
+        }
+    }
+    merged
+}
+
+/// Generate all aligned slot timestamps within the merged ranges.
+fn generate_slots(merged: &[AlignedRange]) -> BTreeSet<DateTime<Utc>> {
+    let mut slots = BTreeSet::new();
+    for range in merged {
+        let mut t = range.start;
+        while t < range.end {
+            slots.insert(t);
+            t += Duration::seconds(SLOT_SECONDS);
+        }
+    }
+    slots
+}
+
+// ── Handler ─────────────────────────────────────────────────────────
+
+/// `POST /api/timeline`
+///
+/// Accepts a JSON body with multiple time ranges. Returns per-subject
+/// enrollment totals bucketed into 15-minute slots. Only courses whose
+/// meeting schedule overlaps a slot contribute to that slot's count.
+pub(crate) async fn timeline(
+    State(state): State<AppState>,
+    Json(body): Json<TimelineRequest>,
+) -> Result<Json<TimelineResponse>, TimelineError> {
+    // ── Validate ────────────────────────────────────────────────────
+    if body.ranges.is_empty() {
+        return Err(TimelineError::bad_request("At least one range is required"));
+    }
+    if body.ranges.len() > MAX_RANGES {
+        return Err(TimelineError::bad_request(format!(
+            "Too many ranges (max {MAX_RANGES})"
+        )));
+    }
+
+    let mut aligned: Vec<AlignedRange> = Vec::with_capacity(body.ranges.len());
+    for r in &body.ranges {
+        if r.end <= r.start {
+            return Err(TimelineError::bad_request(format!(
+                "Range end ({}) must be after start ({})",
+                r.end, r.start
+            )));
+        }
+        let span = r.end - r.start;
+        if span > MAX_RANGE_SPAN {
+            return Err(TimelineError::bad_request(format!(
+                "Range span ({} hours) exceeds maximum ({} hours)",
+                span.num_hours(),
+                MAX_RANGE_SPAN.num_hours()
+            )));
+        }
+        aligned.push(AlignedRange {
+            start: align_floor(r.start),
+            end: align_ceil(r.end),
+        });
+    }
+
+    let merged = merge_ranges(aligned);
+
+    // Validate total span
+    let total_span: Duration = merged.iter().map(|r| r.end - r.start).sum();
+    if total_span > MAX_TOTAL_SPAN {
+        return Err(TimelineError::bad_request(format!(
+            "Total time span ({} hours) exceeds maximum ({} hours)",
+            total_span.num_hours(),
+            MAX_TOTAL_SPAN.num_hours()
+        )));
+    }
+
+    // ── Get cached schedule data (ISR: stale-while-revalidate) ───────
+    state.schedule_cache.ensure_fresh();
+    let snapshot = state.schedule_cache.snapshot();
+
+    // ── Build per-slot enrollment by filtering on meeting times ──────
+    let slot_times = generate_slots(&merged);
+    let mut all_subjects: BTreeSet<String> = BTreeSet::new();
+
+    let slots: Vec<TimelineSlot> = slot_times
+        .into_iter()
+        .map(|utc_time| {
+            // Convert UTC slot to Central time for local day-of-week and time-of-day
+            let local = utc_time.with_timezone(&Central);
+            let local_date = local.date_naive();
+            let local_time = local.time();
+            let weekday = local.weekday();
+            let wday_bit = weekday_bit(weekday);
+            let slot_start_minutes = time_to_minutes(local_time);
+            let slot_end_minutes = slot_start_minutes + SLOT_MINUTES;
+
+            let mut subject_totals: BTreeMap<String, i64> = BTreeMap::new();
+
+            for course in &snapshot.courses {
+                let active = course.schedules.iter().any(|s| {
+                    s.active_during(local_date, wday_bit, slot_start_minutes, slot_end_minutes)
+                });
+                if active {
+                    *subject_totals
+                        .entry(course.subject.clone())
+                        .or_default() += course.enrollment as i64;
+                }
+            }
+
+            all_subjects.extend(subject_totals.keys().cloned());
+
+            TimelineSlot {
+                time: utc_time,
+                subjects: subject_totals,
+            }
+        })
+        .collect();
+
+    let subjects: Vec<String> = all_subjects.into_iter().collect();
+
+    Ok(Json(TimelineResponse { slots, subjects }))
+}
+
+/// Convert a `NaiveTime` to minutes since midnight.
+fn time_to_minutes(t: NaiveTime) -> u16 {
+    (t.hour() * 60 + t.minute()) as u16
+}

web/src/lib/api.ts

@@ -135,6 +135,29 @@ export interface MetricsParams {
   limit?: number;
 }
 
+/** A time range for timeline queries (ISO-8601 strings). */
+export interface TimelineRange {
+  start: string;
+  end: string;
+}
+
+/** Request body for POST /api/timeline. */
+export interface TimelineRequest {
+  ranges: TimelineRange[];
+}
+
+/** A single 15-minute slot returned by the timeline API. */
+export interface TimelineSlot {
+  time: string;
+  subjects: Record<string, number>;
+}
+
+/** Response from POST /api/timeline. */
+export interface TimelineResponse {
+  slots: TimelineSlot[];
+  subjects: string[];
+}
+
 export interface SearchParams {
   term: string;
   subjects?: string[];
@@ -297,6 +320,13 @@ export class BannerApiClient {
   /** Stored `Last-Modified` value for audit log conditional requests. */
   private _auditLastModified: string | null = null;
 
+  async getTimeline(ranges: TimelineRange[]): Promise<TimelineResponse> {
+    return this.request<TimelineResponse>("/timeline", {
+      method: "POST",
+      body: { ranges } satisfies TimelineRequest,
+    });
+  }
+
   async getMetrics(params?: MetricsParams): Promise<MetricsResponse> {
     const query = new URLSearchParams();
     if (params?.course_id !== undefined) query.set("course_id", String(params.course_id));

web/src/lib/components/TimelineCanvas.svelte

@@ -2,7 +2,6 @@
 import { onMount } from "svelte";
 import { scaleTime, scaleLinear } from "d3-scale";
 
-import { SUBJECTS, type Subject } from "$lib/timeline/data";
 import type { TimeSlot, ChartContext } from "$lib/timeline/types";
 import {
   PADDING,
@@ -125,12 +124,31 @@ let pointerOverCanvas = false;
 
 // ── Drawer ──────────────────────────────────────────────────────────
 let drawerOpen = $state(false);
-let enabledSubjects: Set<Subject> = $state(new Set(SUBJECTS));
+// Start with an empty set — subjects are populated dynamically from the API.
+let enabledSubjects: Set<string> = $state(new Set());
 
 // ── Data store ──────────────────────────────────────────────────────
 const store = createTimelineStore();
 let data: TimeSlot[] = $derived(store.data);
-let activeSubjects = $derived(SUBJECTS.filter((s) => enabledSubjects.has(s)));
+let allSubjects: string[] = $derived(store.subjects);
+
+// Auto-enable newly discovered subjects.
+$effect(() => {
+  const storeSubjects = store.subjects;
+  const next = new Set(enabledSubjects);
+  let changed = false;
+  for (const s of storeSubjects) {
+    if (!next.has(s)) {
+      next.add(s);
+      changed = true;
+    }
+  }
+  if (changed) {
+    enabledSubjects = next;
+  }
+});
+
+let activeSubjects = $derived(allSubjects.filter((s) => enabledSubjects.has(s)));
 
 // ── Derived layout ──────────────────────────────────────────────────
 let viewStart = $derived(viewCenter - viewSpan / 2);
@@ -151,7 +169,7 @@ let yScale = scaleLinear()
   .range([0, 1]);
 
 // ── Subject toggling ────────────────────────────────────────────────
-function toggleSubject(subject: Subject) {
+function toggleSubject(subject: string) {
   const next = new Set(enabledSubjects);
   if (next.has(subject)) next.delete(subject);
   else next.add(subject);
@@ -159,7 +177,7 @@ function toggleSubject(subject: Subject) {
 }
 
 function enableAll() {
-  enabledSubjects = new Set(SUBJECTS);
+  enabledSubjects = new Set(allSubjects);
 }
 
 function disableAll() {
@@ -192,7 +210,7 @@ function render() {
   };
 
   const visible = getVisibleSlots(data, viewStart, viewEnd);
-  const visibleStack = stackVisibleSlots(visible, enabledSubjects, animMap);
+  const visibleStack = stackVisibleSlots(visible, allSubjects, enabledSubjects, animMap);
 
   drawGrid(chart);
   drawHoverColumn(chart, visibleStack, hoverSlotTime);
@@ -585,8 +603,9 @@ function tick(timestamp: number) {
 // ── Animation sync ──────────────────────────────────────────────────
 $effect(() => {
   const slots = data;
+  const subs = allSubjects;
   const enabled = enabledSubjects;
-  syncAnimTargets(animMap, slots, enabled);
+  syncAnimTargets(animMap, slots, subs, enabled);
 });
 
 // Request data whenever the visible window changes.
@@ -625,7 +644,7 @@ onMount(() => {
     class:cursor-grabbing={isDragging}
     style="display: block; touch-action: none;"
     tabindex="0"
-    aria-label="Interactive class schedule timeline chart"
+    aria-label="Interactive enrollment timeline chart"
     onpointerdown={(e) => { canvasEl?.focus(); onPointerDown(e); }}
     onpointermove={onPointerMove}
     onpointerup={onPointerUp}
@@ -638,6 +657,7 @@ onMount(() => {
 
   <TimelineDrawer
     bind:open={drawerOpen}
+    subjects={allSubjects}
     {enabledSubjects}
     {followEnabled}
     onToggleSubject={toggleSubject}

web/src/lib/components/TimelineDrawer.svelte

@@ -1,13 +1,14 @@
 <script lang="ts">
 import { Filter, X } from "@lucide/svelte";
-import { SUBJECTS, SUBJECT_COLORS, type Subject } from "$lib/timeline/data";
+import { getSubjectColor } from "$lib/timeline/data";
 import { DRAWER_WIDTH } from "$lib/timeline/constants";
 
 interface Props {
   open: boolean;
-  enabledSubjects: Set<Subject>;
+  subjects: readonly string[];
+  enabledSubjects: Set<string>;
   followEnabled: boolean;
-  onToggleSubject: (subject: Subject) => void;
+  onToggleSubject: (subject: string) => void;
   onEnableAll: () => void;
   onDisableAll: () => void;
   onResumeFollow: () => void;
@@ -15,6 +16,7 @@ interface Props {
 
 let {
   open = $bindable(),
+  subjects,
   enabledSubjects,
   followEnabled,
   onToggleSubject,
@@ -109,8 +111,9 @@ function onKeyDown(e: KeyboardEvent) {
         </div>
       </div>
       <div class="space-y-0.5">
-        {#each SUBJECTS as subject}
+        {#each subjects as subject}
           {@const enabled = enabledSubjects.has(subject)}
+          {@const color = getSubjectColor(subject)}
           <button
             class="flex items-center gap-2 w-full px-1.5 py-1 rounded text-xs
               hover:bg-muted/50 transition-colors cursor-pointer text-left"
@@ -118,7 +121,7 @@ function onKeyDown(e: KeyboardEvent) {
           >
             <span
               class="inline-block w-3 h-3 rounded-sm shrink-0 transition-opacity"
-              style="background: {SUBJECT_COLORS[subject]}; opacity: {enabled ? 1 : 0.2};"
+              style="background: {color}; opacity: {enabled ? 1 : 0.2};"
             ></span>
             <span
               class="transition-opacity {enabled

web/src/lib/components/TimelineTooltip.svelte

@@ -1,6 +1,6 @@
 <script lang="ts">
 import { timeFormat } from "d3-time-format";
-import { SUBJECT_COLORS, type Subject } from "$lib/timeline/data";
+import { getSubjectColor } from "$lib/timeline/data";
 import type { TimeSlot } from "$lib/timeline/types";
 import { enabledTotalClasses } from "$lib/timeline/viewport";
 
@@ -9,7 +9,7 @@ interface Props {
   x: number;
   y: number;
   slot: TimeSlot | null;
-  activeSubjects: readonly Subject[];
+  activeSubjects: readonly string[];
 }
 
 let { visible, x, y, slot, activeSubjects }: Props = $props();
@@ -35,7 +35,7 @@ const fmtTime = timeFormat("%-I:%M %p");
             <div class="flex items-center gap-1.5">
               <span
                 class="inline-block w-2 h-2 rounded-sm"
-                style="background: {SUBJECT_COLORS[subject]}"
+                style="background: {getSubjectColor(subject)}"
               ></span>
               <span class="text-muted-foreground">{subject}</span>
             </div>

web/src/lib/timeline/animation.ts

@@ -5,7 +5,6 @@
  * targets. This module owns the AnimMap lifecycle: syncing targets,
  * stepping current values, and pruning offscreen entries.
  */
-import { SUBJECTS, type Subject } from "./data";
 import { VALUE_EASE, MAXY_EASE, SETTLE_THRESHOLD, MIN_MAXY } from "./constants";
 import type { AnimEntry, TimeSlot } from "./types";
 
@@ -20,11 +19,15 @@ export function createAnimMap(): AnimMap {
  * Sync animMap targets from data + filter state.
  * New slots start at current=0 so they animate in from the baseline.
  * Disabled subjects get target=0 so they animate out.
+ *
+ * @param subjects - the full list of known subject codes
+ * @param enabledSubjects - subjects currently toggled on
  */
 export function syncAnimTargets(
   animMap: AnimMap,
   slots: TimeSlot[],
-  enabledSubjects: Set<Subject>
+  subjects: readonly string[],
+  enabledSubjects: Set<string>
 ): void {
   for (const slot of slots) {
     const timeMs = slot.time.getTime();
@@ -34,7 +37,7 @@ export function syncAnimTargets(
       animMap.set(timeMs, subjectMap);
     }
 
-    for (const subject of SUBJECTS) {
+    for (const subject of subjects) {
       const realValue = enabledSubjects.has(subject) ? slot.subjects[subject] || 0 : 0;
       const entry = subjectMap.get(subject);
       if (entry) {

web/src/lib/timeline/data.ts

@@ -1,122 +1,78 @@
 /**
- * Data types, constants, and deterministic slot generation for the class timeline.
- * Each 15-minute slot is seeded by its timestamp, so the same slot always produces
- * identical data regardless of when or in what order it's fetched.
+ * Subject color palette for the timeline chart.
+ *
+ * Subjects are dynamic (coming from the API), so we assign colors from
+ * a fixed palette based on a deterministic hash of the subject code.
+ * Known high-enrollment subjects get hand-picked colors for familiarity.
  */
-import { SLOT_INTERVAL_MS } from "./constants";
-import type { TimeSlot } from "./types";
 
-export type { TimeSlot };
-
-export const SUBJECTS = [
-  "CS",
-  "MATH",
-  "BIO",
-  "ENG",
-  "PHYS",
-  "HIST",
-  "CHEM",
-  "PSY",
-  "ECE",
-  "ART",
-] as const;
-
-export type Subject = (typeof SUBJECTS)[number];
-
-/** Subject colors — distinct, accessible palette */
-export const SUBJECT_COLORS: Record<Subject, string> = {
+/** Hand-picked colors for common UTSA subject codes. */
+const KNOWN_SUBJECT_COLORS: Record<string, string> = {
   CS: "#6366f1", // indigo
-  MATH: "#f59e0b", // amber
+  MAT: "#f59e0b", // amber
   BIO: "#10b981", // emerald
   ENG: "#ef4444", // red
-  PHYS: "#3b82f6", // blue
-  HIST: "#8b5cf6", // violet
-  CHEM: "#f97316", // orange
+  PHY: "#3b82f6", // blue
+  HIS: "#8b5cf6", // violet
+  CHE: "#f97316", // orange
   PSY: "#ec4899", // pink
   ECE: "#14b8a6", // teal
   ART: "#a855f7", // purple
+  ACC: "#84cc16", // lime
+  FIN: "#06b6d4", // cyan
+  MUS: "#e11d48", // rose
+  POL: "#d946ef", // fuchsia
+  SOC: "#22d3ee", // sky
+  KIN: "#4ade80", // green
+  IS: "#fb923c", // light orange
+  STA: "#818cf8", // light indigo
+  MGT: "#fbbf24", // yellow
+  MKT: "#2dd4bf", // teal-light
 };
 
 /**
- * Bell-curve-like distribution centered at a given hour.
- * Returns a value 0..1 representing relative class density.
+ * Extended palette for subjects that don't have a hand-picked color.
+ * These are chosen to be visually distinct from each other.
  */
-function bellCurve(hour: number, center: number, spread: number): number {
-  const x = (hour - center) / spread;
-  return Math.exp(-0.5 * x * x);
-}
+const FALLBACK_PALETTE = [
+  "#f472b6", // pink-400
+  "#60a5fa", // blue-400
+  "#34d399", // emerald-400
+  "#fbbf24", // amber-400
+  "#a78bfa", // violet-400
+  "#fb7185", // rose-400
+  "#38bdf8", // sky-400
+  "#4ade80", // green-400
+  "#facc15", // yellow-400
+  "#c084fc", // purple-400
+  "#f87171", // red-400
+  "#2dd4bf", // teal-400
+  "#fb923c", // orange-400
+  "#818cf8", // indigo-400
+  "#a3e635", // lime-400
+  "#22d3ee", // cyan-400
+];
 
-/**
- * Each subject has characteristic scheduling patterns:
- * peak hours, relative popularity, and spread.
- */
-const SUBJECT_PROFILES: Record<Subject, { peaks: number[]; weight: number; spread: number }> = {
-  CS: { peaks: [10, 14, 16], weight: 12, spread: 2.0 },
-  MATH: { peaks: [8, 10, 13], weight: 10, spread: 1.8 },
-  BIO: { peaks: [9, 11, 14], weight: 8, spread: 1.5 },
-  ENG: { peaks: [9, 11, 14, 16], weight: 7, spread: 2.2 },
-  PHYS: { peaks: [8, 13, 15], weight: 6, spread: 1.6 },
-  HIST: { peaks: [10, 13, 15], weight: 5, spread: 2.0 },
-  CHEM: { peaks: [8, 10, 14], weight: 6, spread: 1.5 },
-  PSY: { peaks: [11, 14, 16], weight: 7, spread: 2.0 },
-  ECE: { peaks: [9, 13, 15], weight: 5, spread: 1.8 },
-  ART: { peaks: [10, 14, 17], weight: 4, spread: 2.5 },
-};
-
-/**
- * Seeded pseudo-random number generator (LCG) for reproducible data.
- */
-function seededRandom(seed: number): () => number {
-  let s = seed;
-  return () => {
-    s = (s * 1664525 + 1013904223) & 0xffffffff;
-    return (s >>> 0) / 0xffffffff;
-  };
-}
-
-/**
- * Integer hash so adjacent slot timestamps produce very different seeds.
- */
-function hashTimestamp(ms: number): number {
-  let h = ms | 0;
-  h = ((h >> 16) ^ h) * 0x45d9f3b;
-  h = ((h >> 16) ^ h) * 0x45d9f3b;
-  h = (h >> 16) ^ h;
-  return h >>> 0;
-}
-
-/** Generate a single TimeSlot for the given aligned timestamp. */
-function generateSlot(timeMs: number): TimeSlot {
-  const rand = seededRandom(hashTimestamp(timeMs));
-  const time = new Date(timeMs);
-  const hour = time.getHours() + time.getMinutes() / 60;
-
-  const subjects = {} as Record<Subject, number>;
-  for (const subject of SUBJECTS) {
-    const profile = SUBJECT_PROFILES[subject];
-    let density = 0;
-    for (const peak of profile.peaks) {
-      density += bellCurve(hour, peak, profile.spread);
-    }
-    const base = density * profile.weight;
-    const noise = (rand() - 0.5) * 2;
-    subjects[subject] = Math.max(0, Math.round(base + noise));
+/** Simple string hash for deterministic color assignment. */
+function hashCode(str: string): number {
+  let hash = 0;
+  for (let i = 0; i < str.length; i++) {
+    hash = ((hash << 5) - hash + str.charCodeAt(i)) | 0;
   }
-
-  return { time, subjects };
+  return Math.abs(hash);
 }
 
-/**
- * Generate TimeSlots covering [startMs, endMs], aligned to 15-minute boundaries.
- * Each slot is deterministically seeded by its timestamp.
- */
-export function generateSlots(startMs: number, endMs: number): TimeSlot[] {
-  const alignedStart = Math.floor(startMs / SLOT_INTERVAL_MS) * SLOT_INTERVAL_MS;
-  const alignedEnd = Math.ceil(endMs / SLOT_INTERVAL_MS) * SLOT_INTERVAL_MS;
+/** Cache of assigned colors to avoid re-computing. */
+const colorCache = new Map<string, string>();
 
-  const slots: TimeSlot[] = [];
-  for (let t = alignedStart; t <= alignedEnd; t += SLOT_INTERVAL_MS) {
-    slots.push(generateSlot(t));
-  }
-  return slots;
+/** Get a consistent color for any subject code. */
+export function getSubjectColor(subject: string): string {
+  const cached = colorCache.get(subject);
+  if (cached) return cached;
+
+  const color =
+    KNOWN_SUBJECT_COLORS[subject] ?? FALLBACK_PALETTE[hashCode(subject) % FALLBACK_PALETTE.length];
+
+  colorCache.set(subject, color);
+  return color;
 }

web/src/lib/timeline/renderer.ts

@@ -7,7 +7,7 @@
 import { stack, area, curveMonotoneX, type Series } from "d3-shape";
 import { timeFormat } from "d3-time-format";
 
-import { SUBJECT_COLORS, type Subject } from "./data";
+import { getSubjectColor } from "./data";
 import type { AnimMap } from "./animation";
 import { getStackSubjects } from "./viewport";
 import type { ChartContext, TimeSlot } from "./types";
@@ -55,22 +55,31 @@ export function chooseTickCount(viewSpan: number): number {
  * Stack only the visible slice using *animated* values so transitions
  * between filter/data states are smooth. Includes subjects that are
  * still animating out so removal is gradual.
+ *
+ * @param allSubjects - full set of known subject codes
  */
 export function stackVisibleSlots(
   visible: TimeSlot[],
-  enabledSubjects: Set<Subject>,
+  allSubjects: readonly string[],
+  enabledSubjects: Set<string>,
   animMap: AnimMap
 ): VisibleStack {
   if (visible.length === 0) return [];
 
-  const stackKeys = getStackSubjects(visible, enabledSubjects, animMap, SETTLE_THRESHOLD);
+  const stackKeys = getStackSubjects(
+    visible,
+    allSubjects,
+    enabledSubjects,
+    animMap,
+    SETTLE_THRESHOLD
+  );
   if (stackKeys.length === 0) return [];
 
   // Build synthetic slots with animated current values.
   const animatedSlots: TimeSlot[] = visible.map((slot) => {
     const timeMs = slot.time.getTime();
     const subjectMap = animMap.get(timeMs);
-    const subjects = {} as Record<Subject, number>;
+    const subjects: Record<string, number> = {};
     for (const subject of stackKeys) {
       const entry = subjectMap?.get(subject);
       subjects[subject] = entry ? entry.current : slot.subjects[subject] || 0;
@@ -80,7 +89,7 @@ export function stackVisibleSlots(
 
   const gen = stack<TimeSlot>()
     .keys(stackKeys)
-    .value((d, key) => d.subjects[key as Subject] || 0);
+    .value((d, key) => d.subjects[key] || 0);
   return gen(animatedSlots);
 }
 
@@ -187,8 +196,8 @@ export function drawStackedArea(chart: ChartContext, visibleStack: VisibleStack)
 
   for (let i = visibleStack.length - 1; i >= 0; i--) {
     const layer = visibleStack[i];
-    const subject = layer.key as Subject;
-    const color = SUBJECT_COLORS[subject];
+    const subject = layer.key;
+    const color = getSubjectColor(subject);
 
     ctx.beginPath();
     area<StackPoint>()

web/src/lib/timeline/store.svelte.ts

@@ -3,10 +3,9 @@
  *
  * Tracks which time ranges have already been fetched and only requests
  * the missing segments when the view expands into unloaded territory.
- * Fetches are throttled so rapid panning/zooming doesn't flood the
- * (currently mock) API.
+ * Fetches are throttled so rapid panning/zooming doesn't flood the API.
  */
-import { generateSlots } from "./data";
+import { client, type TimelineRange } from "$lib/api";
 import { SLOT_INTERVAL_MS } from "./constants";
 import type { TimeSlot } from "./types";
 
@@ -16,20 +15,6 @@ type Range = [start: number, end: number];
 const FETCH_THROTTLE_MS = 500;
 const BUFFER_RATIO = 0.15;
 
-// Mock network latency bounds (ms).
-const MOCK_DELAY_MIN = 40;
-const MOCK_DELAY_MAX = 120;
-
-/**
- * Simulate an API call that returns slots for an arbitrary time range.
- * The delay makes loading behaviour visible during development.
- */
-async function mockFetch(startMs: number, endMs: number): Promise<TimeSlot[]> {
-  const delay = MOCK_DELAY_MIN + Math.random() * (MOCK_DELAY_MAX - MOCK_DELAY_MIN);
-  await new Promise((r) => setTimeout(r, delay));
-  return generateSlots(startMs, endMs);
-}
-
 /** Align a timestamp down to the nearest slot boundary. */
 function alignFloor(ms: number): number {
   return Math.floor(ms / SLOT_INTERVAL_MS) * SLOT_INTERVAL_MS;
@@ -84,6 +69,24 @@ function mergeRange(ranges: Range[], added: Range): Range[] {
   return merged;
 }
 
+/**
+ * Fetch timeline data for the given gap ranges from the API.
+ * Converts gap ranges into the API request format.
+ */
+async function fetchFromApi(gaps: Range[]): Promise<TimeSlot[]> {
+  const ranges: TimelineRange[] = gaps.map(([start, end]) => ({
+    start: new Date(start).toISOString(),
+    end: new Date(end).toISOString(),
+  }));
+
+  const response = await client.getTimeline(ranges);
+
+  return response.slots.map((slot) => ({
+    time: new Date(slot.time),
+    subjects: slot.subjects,
+  }));
+}
+
 /**
  * Create a reactive timeline store.
  *
@@ -93,6 +96,9 @@ function mergeRange(ranges: Range[], added: Range): Range[] {
  *
  * The `data` getter returns a sorted `TimeSlot[]` that reactively
  * updates as new segments arrive.
+ *
+ * The `subjects` getter returns the sorted list of all subject codes
+ * seen so far across all fetched data.
  */
 export function createTimelineStore() {
   // All loaded slots keyed by aligned timestamp (ms).
@@ -101,6 +107,9 @@ export function createTimelineStore() {
   // Sorted, non-overlapping list of fetched ranges.
   let loadedRanges: Range[] = [];
 
+  // All subject codes observed across all fetched data.
+  let knownSubjects: Set<string> = $state(new Set());
+
   let throttleTimer: ReturnType<typeof setTimeout> | undefined;
   let pendingStart = 0;
   let pendingEnd = 0;
@@ -112,18 +121,28 @@ export function createTimelineStore() {
     [...slotMap.values()].sort((a, b) => a.time.getTime() - b.time.getTime())
   );
 
+  // Sorted subject list derived from the known subjects set.
+  const subjects: string[] = $derived([...knownSubjects].sort());
+
   async function fetchGaps(start: number, end: number): Promise<void> {
     const gaps = findGaps(start, end, loadedRanges);
     if (gaps.length === 0) return;
 
-    // Fetch all gap segments in parallel.
-    const results = await Promise.all(gaps.map(([gs, ge]) => mockFetch(gs, ge)));
+    let slots: TimeSlot[];
+    try {
+      slots = await fetchFromApi(gaps);
+    } catch (err) {
+      console.error("Timeline fetch failed:", err);
+      return;
+    }
 
     // Merge results into the slot map.
     const next = new Map(slotMap);
-    for (const slots of results) {
-      for (const slot of slots) {
-        next.set(slot.time.getTime(), slot);
+    const nextSubjects = new Set(knownSubjects);
+    for (const slot of slots) {
+      next.set(slot.time.getTime(), slot);
+      for (const subject of Object.keys(slot.subjects)) {
+        nextSubjects.add(subject);
       }
     }
 
@@ -132,8 +151,9 @@ export function createTimelineStore() {
       loadedRanges = mergeRange(loadedRanges, gap);
     }
 
-    // Single reactive assignment.
+    // Single reactive assignments.
     slotMap = next;
+    knownSubjects = nextSubjects;
   }
 
   /**
@@ -173,6 +193,9 @@ export function createTimelineStore() {
     get data() {
       return data;
     },
+    get subjects() {
+      return subjects;
+    },
     requestRange,
     dispose,
   };

web/src/lib/timeline/types.ts

@@ -1,16 +1,16 @@
 /**
  * Shared types for the timeline feature.
+ *
+ * Subjects are dynamic strings (actual Banner subject codes like "CS",
+ * "MAT", "BIO") rather than a fixed enum — the set of subjects comes
+ * from the API response.
  */
 import type { ScaleLinear, ScaleTime } from "d3-scale";
 
-import type { Subject } from "./data";
-
-export type { Subject };
-
-/** A single 15-minute time slot with per-subject class counts. */
+/** A single 15-minute time slot with per-subject enrollment totals. */
 export interface TimeSlot {
   time: Date;
-  subjects: Record<Subject, number>;
+  subjects: Record<string, number>;
 }
 
 /** Lerped animation entry for a single subject within a slot. */

web/src/lib/timeline/viewport.ts

@@ -3,7 +3,6 @@
  * hit-testing, and snapping for the timeline canvas.
  */
 import { SLOT_INTERVAL_MS, RENDER_MARGIN_SLOTS } from "./constants";
-import { SUBJECTS, type Subject } from "./data";
 import type { TimeSlot } from "./types";
 
 /**
@@ -55,8 +54,8 @@ export function snapToSlot(timeMs: number): number {
   return Math.floor(timeMs / SLOT_INTERVAL_MS) * SLOT_INTERVAL_MS;
 }
 
-/** Sum of class counts for enabled subjects in a slot. */
-export function enabledTotalClasses(slot: TimeSlot, activeSubjects: readonly Subject[]): number {
+/** Sum of enrollment counts for enabled subjects in a slot. */
+export function enabledTotalClasses(slot: TimeSlot, activeSubjects: readonly string[]): number {
   let sum = 0;
   for (const s of activeSubjects) {
     sum += slot.subjects[s] || 0;
@@ -67,15 +66,18 @@ export function enabledTotalClasses(slot: TimeSlot, activeSubjects: readonly Sub
 /**
  * Determine which subjects to include in the stack: all enabled subjects
  * plus any disabled subjects still animating out (current > threshold).
+ *
+ * @param allSubjects - the full set of known subject codes
  */
 export function getStackSubjects(
   visible: TimeSlot[],
-  enabledSubjects: Set<Subject>,
+  allSubjects: readonly string[],
+  enabledSubjects: Set<string>,
   animMap: Map<number, Map<string, { current: number }>>,
   settleThreshold: number
-): Subject[] {
-  const subjects: Subject[] = [];
-  for (const subject of SUBJECTS) {
+): string[] {
+  const subjects: string[] = [];
+  for (const subject of allSubjects) {
     if (enabledSubjects.has(subject)) {
       subjects.push(subject);
       continue;