fix(webapp): use stable chart bucket timestamps (#4717)

## Summary

Uses explicit bucket timestamps when rendering usage charts instead of
anchoring missing timestamps to the current render time. Tooltips now
remain stable across rerenders, and examples use a deterministic
timestamp.

apps/webapp/app/components/metrics/MiniLineChart.tsx

@@ -44,7 +44,7 @@ export type MiniLineChartProps = {
   throttled?: number[];
   /** Tooltip wording for the overlay buckets. Null omits the overlay line. */
   overlayLabel?: string | null;
-  /** Epoch ms of the first bucket's start. When omitted, the last bucket is anchored to now. */
+  /** Epoch ms of the first bucket's start. */
   bucketStartMs?: number;
   /** Width of each bucket in ms. Defaults to one hour. */
   bucketIntervalMs?: number;
@@ -92,7 +92,12 @@ export function MiniLineChart({
   showPeak = true,
 }: MiniLineChartProps) {
   const hasPeakOverride = peakOverride !== undefined;
-  if (!data || data.length === 0 || (data.every((v) => v === 0) && !hasPeakOverride)) {
+  if (
+    !data ||
+    data.length === 0 ||
+    bucketStartMs === undefined ||
+    (data.every((v) => v === 0) && !hasPeakOverride)
+  ) {
     return <span className="text-text-dimmed">–</span>;
   }
 
@@ -103,11 +108,9 @@ export function MiniLineChart({
   const max = Math.max(...data);
   const peak = peakOverride ?? max;
 
-  // Map each bucket to a dated point so the tooltip can show the window it represents. Buckets are
-  // `intervalMs` wide; if the caller didn't pass the first bucket's start, anchor the last bucket to
-  // now (hourly default).
+  // Map each bucket to a dated point so the tooltip can show the window it represents.
   const intervalMs = bucketIntervalMs ?? 3600_000;
-  const startMs = bucketStartMs ?? Date.now() - (data.length - 1) * intervalMs;
+  const startMs = bucketStartMs;
   const chartData: MiniLineChartDatum[] = data.map((count, i) => {
     const t = throttled?.[i] ?? 0;
     // Extend the mask one bucket forward (a segment needs both endpoints non-null), so even a

apps/webapp/app/components/primitives/UsageSparkline.tsx

@@ -19,8 +19,8 @@ type UnitLabel = { singular: string; plural: string };
 export type UsageSparklineProps = {
   /** Equal-width time buckets, oldest first. */
   data?: number[];
-  /** Epoch ms of the first bucket's start. When omitted, the last bucket is anchored to now. */
-  bucketStartMs?: number;
+  /** Epoch ms of the first bucket's start. */
+  bucketStartMs: number;
   /** Width of each bucket in ms. Defaults to one hour. */
   bucketIntervalMs?: number;
   /** Bar colour. Defaults to blue. */
@@ -64,11 +64,9 @@ export function UsageSparkline({
   const total = totalOverride ?? data.reduce((a, b) => a + b, 0);
   const max = Math.max(...data);
 
-  // Map each bucket to a dated point so the tooltip can show the window it
-  // represents. Buckets are `intervalMs` wide; if the caller didn't pass the
-  // first bucket's start, anchor the last bucket to now (hourly default).
+  // Map each bucket to a dated point so the tooltip can show the window it represents.
   const intervalMs = bucketIntervalMs ?? 3600_000;
-  const startMs = bucketStartMs ?? Date.now() - (data.length - 1) * intervalMs;
+  const startMs = bucketStartMs;
   const chartData: UsageDatum[] = data.map((count, i) => ({
     date: new Date(startMs + i * intervalMs),
     count,

apps/webapp/app/routes/storybook.charts/route.tsx

@@ -21,6 +21,7 @@ import SegmentedControl from "~/components/primitives/SegmentedControl";
 // Date formatters for chart display
 const xAxisTickFormatter = (value: string) => formatISODate(value);
 const tooltipLabelFormatter = (label: string) => formatISODateLong(label);
+const MINI_LINE_BUCKET_START_MS = Date.UTC(2025, 0, 1);
 
 /**
  * Helper function to filter chart data by date range.
@@ -361,6 +362,7 @@ function ChartsDashboard() {
                   <td className="py-1.5">
                     <MiniLineChart
                       data={API_DATA.miniLineData}
+                      bucketStartMs={MINI_LINE_BUCKET_START_MS}
                       peak={Math.max(...API_DATA.miniLineData)}
                       unitLabel={{ singular: "queued", plural: "queued" }}
                       color="var(--color-tasks)"
@@ -372,6 +374,7 @@ function ChartsDashboard() {
                   <td className="py-1.5">
                     <MiniLineChart
                       data={API_DATA.miniLineThrottledData}
+                      bucketStartMs={MINI_LINE_BUCKET_START_MS}
                       throttled={API_DATA.miniLineThrottledBuckets}
                       peak={Math.max(...API_DATA.miniLineThrottledData)}
                       unitLabel={{ singular: "queued", plural: "queued" }}