fix(panel): days-view timeseries charts show dates, not '02:00' (#622)

formatBucket guessed hourly vs daily from the timestamp string
(bucket.endsWith('T00:00:00.000Z')) and then rendered LOCAL getHours(), so
every daily midnight-UTC bucket rendered as the viewer's local hour — '02:00'
at UTC+2 — across the 7d/30d/90d windows. Granularity is now derived from the
data's own bucket spacing (bucketGranularity: min gap >2h = daily) and daily
buckets render as a short UTC date. The 24h/hourly view is unchanged. Added
minTickGap so the 90d axis doesn't crowd.

Co-authored-by: Renn F <rennf93@users.noreply.github.com>

panel/src/components/agents/agent-activity-panel.tsx

@@ -23,7 +23,7 @@ import { GitBranch, BookOpen } from "lucide-react";
 import { useUsageTimeSeries } from "@/hooks/use-usage";
 import { useWorkSessions } from "@/hooks/use-work-sessions";
 import { useAgentJournalEntries } from "@/hooks/use-journals";
-import { formatTokens, formatBucket } from "@/lib/format";
+import { formatTokens, formatBucket, bucketGranularity } from "@/lib/format";
 import { chartTooltipStyle } from "@/components/charts/chart-tooltip";
 import { WorkSessionStatus, type UsageTimePoint } from "@/types";
 
@@ -109,6 +109,7 @@ export function AgentActivityPanel({
   const hasTokens = (series ?? []).some(
     (p: UsageTimePoint) => p.total_tokens > 0,
   );
+  const granularity = bucketGranularity((series ?? []).map((p) => p.bucket));
 
   return (
     <div className="grid gap-4 md:grid-cols-2">
@@ -145,7 +146,7 @@ export function AgentActivityPanel({
                 </defs>
                 <XAxis
                   dataKey="bucket"
-                  tickFormatter={formatBucket}
+                  tickFormatter={(b) => formatBucket(String(b), granularity)}
                   tick={{ fontSize: 9 }}
                   axisLine={false}
                   tickLine={false}
@@ -163,7 +164,9 @@ export function AgentActivityPanel({
                     formatTokens(typeof value === "number" ? value : 0),
                     "Tokens",
                   ]}
-                  labelFormatter={(label) => formatBucket(String(label))}
+                  labelFormatter={(label) =>
+                    formatBucket(String(label), granularity)
+                  }
                 />
                 <Area
                   type="monotone"

panel/src/components/business/spend-trend-chart.tsx

@@ -12,7 +12,7 @@ import {
 import { Card, CardContent, CardHeader, CardTitle } from "@/components/ui/card";
 import { Skeleton } from "@/components/ui/skeleton";
 import { HelpTip } from "@/components/ui/help-tip";
-import { formatBucket } from "@/lib/format";
+import { formatBucket, bucketGranularity } from "@/lib/format";
 import { chartTooltipStyle } from "@/components/charts/chart-tooltip";
 import type { UsageTimePoint } from "@/types";
 
@@ -31,8 +31,9 @@ function fmtCost(n: number): string {
  * series-shaped endpoint the Overview page's CostTrendChart draws from.
  */
 export function SpendTrendChart({ data, isLoading }: SpendTrendChartProps) {
+  const granularity = bucketGranularity((data ?? []).map((p) => p.bucket));
   const chartData = (data ?? []).map((p) => ({
-    day: formatBucket(p.bucket),
+    day: formatBucket(p.bucket, granularity),
     Spend: p.cost_usd,
   }));
 

panel/src/components/dashboard/cost-trend-chart.tsx

@@ -12,7 +12,7 @@ import {
 import { Card, CardContent, CardHeader, CardTitle } from "@/components/ui/card";
 import { Skeleton } from "@/components/ui/skeleton";
 import { HelpTip } from "@/components/ui/help-tip";
-import { formatBucket } from "@/lib/format";
+import { formatBucket, bucketGranularity } from "@/lib/format";
 import { chartTooltipStyle } from "@/components/charts/chart-tooltip";
 import type { UsageTimePoint } from "@/types";
 
@@ -31,8 +31,9 @@ function fmtCost(n: number): string {
  * the current-period totals in UsageOverviewPanel came from at a glance.
  */
 export function CostTrendChart({ data, isLoading }: CostTrendChartProps) {
+  const granularity = bucketGranularity((data ?? []).map((p) => p.bucket));
   const chartData = (data ?? []).map((p) => ({
-    day: formatBucket(p.bucket),
+    day: formatBucket(p.bucket, granularity),
     Cost: p.cost_usd,
   }));
 

panel/src/components/metrics/usage-time-series-chart.tsx

@@ -14,7 +14,7 @@ import { Card, CardContent, CardHeader, CardTitle } from "@/components/ui/card";
 import { Skeleton } from "@/components/ui/skeleton";
 import { HelpTip } from "@/components/ui/help-tip";
 import { useIsMobile } from "@/hooks/use-is-mobile";
-import { formatTokens, formatBucket } from "@/lib/format";
+import { formatTokens, formatBucket, bucketGranularity } from "@/lib/format";
 import { chartTooltipStyle } from "@/components/charts/chart-tooltip";
 import type { UsageTimePoint } from "@/types";
 
@@ -28,8 +28,9 @@ export function UsageTimeSeriesChart({
   isLoading,
 }: UsageTimeSeriesChartProps) {
   const isMobile = useIsMobile();
+  const granularity = bucketGranularity((data ?? []).map((p) => p.bucket));
   const chartData = (data ?? []).map((p) => ({
-    hour: formatBucket(p.bucket),
+    hour: formatBucket(p.bucket, granularity),
     Input: p.tokens_input,
     Output: p.tokens_output,
   }));
@@ -85,6 +86,7 @@ export function UsageTimeSeriesChart({
                 dataKey="hour"
                 tick={{ fontSize: isMobile ? 9 : 10 }}
                 interval="preserveStartEnd"
+                minTickGap={40}
                 axisLine={false}
                 tickLine={false}
               />

panel/src/lib/__tests__/format.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect } from "vitest";
-import { formatTokens, formatBucket } from "@/lib/format";
+import { formatTokens, formatBucket, bucketGranularity } from "@/lib/format";
 
 describe("formatTokens", () => {
   it("renders raw digits below 1000", () => {
@@ -19,21 +19,47 @@ describe("formatTokens", () => {
   });
 });
 
-describe("formatBucket", () => {
-  it("formats a midnight-UTC daily bucket as MM/DD", () => {
-    // Regression guard: a plain "T00:00:00.000Z" bucket must never be
-    // mistaken for an hourly bucket just because minutes/seconds are 0.
-    expect(formatBucket("2026-07-15T00:00:00.000Z")).toBe(
-      new Date("2026-07-15T00:00:00.000Z").getMonth() +
-        1 +
-        "/" +
-        new Date("2026-07-15T00:00:00.000Z").getDate(),
-    );
+describe("bucketGranularity", () => {
+  it("hourly when buckets sit ~1h apart", () => {
+    expect(
+      bucketGranularity([
+        "2026-07-15T00:00:00Z",
+        "2026-07-15T01:00:00Z",
+        "2026-07-15T02:00:00Z",
+      ]),
+    ).toBe("hour");
   });
 
-  it("formats a non-midnight hourly bucket as HH:00", () => {
+  it("daily when buckets sit ~24h apart", () => {
+    expect(
+      bucketGranularity([
+        "2026-07-13T00:00:00Z",
+        "2026-07-14T00:00:00Z",
+        "2026-07-15T00:00:00Z",
+      ]),
+    ).toBe("day");
+  });
+
+  it("defaults to hourly with too few points to tell", () => {
+    expect(bucketGranularity(["2026-07-15T00:00:00Z"])).toBe("hour");
+  });
+});
+
+describe("formatBucket", () => {
+  it("renders a daily bucket as a date, never a bare local hour", () => {
+    // Regression: a midnight-UTC daily bucket used to render as the viewer's
+    // local hour ("02:00" at UTC+2) for every tick. It must be a date.
+    const out = formatBucket("2026-07-15T00:00:00.000Z", "day");
+    expect(out).not.toMatch(/^\d{2}:00$/);
+    expect(out).toContain("15");
+  });
+
+  it("renders an hourly bucket as HH:00", () => {
     const bucket = "2026-07-15T14:00:00.000Z";
-    const expectedHour = new Date(bucket).getHours().toString().padStart(2, "0");
-    expect(formatBucket(bucket)).toBe(`${expectedHour}:00`);
+    const expectedHour = new Date(bucket)
+      .getHours()
+      .toString()
+      .padStart(2, "0");
+    expect(formatBucket(bucket, "hour")).toBe(`${expectedHour}:00`);
   });
 });

panel/src/lib/format.ts

@@ -9,15 +9,36 @@ export function formatTokens(n: number): string {
   return String(n);
 }
 
-/** Format a usage time-bucket ISO string as "HH:00" (hourly) or "MM/DD" (daily). */
-export function formatBucket(bucket: string): string {
-  const d = new Date(bucket);
-  // If the bucket has a non-zero time component it is an hourly bucket → show HH:00.
-  // Otherwise it is a daily bucket → show MM/DD.
-  const isHourly =
-    d.getMinutes() === 0 && (d.getHours() !== 0 || bucket.includes("T"));
-  if (isHourly && d.getSeconds() === 0 && !bucket.endsWith("T00:00:00.000Z")) {
-    return d.getHours().toString().padStart(2, "0") + ":00";
+const _TWO_HOURS_MS = 2 * 60 * 60 * 1000;
+
+/** Whether a time-bucket series is hourly or daily, from the smallest gap
+ * between consecutive buckets — hourly buckets sit ~1h apart, daily ~24h.
+ * Derived from the data (not the timestamp's string shape or the viewer's
+ * timezone), so it can't misread a midnight-UTC daily bucket as an hour. */
+export function bucketGranularity(buckets: string[]): "hour" | "day" {
+  if (buckets.length < 2) return "hour";
+  const times = buckets.map((b) => new Date(b).getTime()).sort((a, b) => a - b);
+  let minGap = Infinity;
+  for (let i = 1; i < times.length; i++) {
+    minGap = Math.min(minGap, times[i] - times[i - 1]);
   }
-  return d.getMonth() + 1 + "/" + d.getDate();
+  return minGap > _TWO_HOURS_MS ? "day" : "hour";
+}
+
+/** Format one time-bucket: "HH:00" for hourly (viewer-local), a short UTC
+ * date ("Jul 15") for daily. Daily buckets are midnight UTC, so render them in
+ * UTC — otherwise a non-UTC viewer sees the previous day or a bare "02:00". */
+export function formatBucket(
+  bucket: string,
+  granularity: "hour" | "day",
+): string {
+  const d = new Date(bucket);
+  if (granularity === "day") {
+    return d.toLocaleDateString(undefined, {
+      month: "short",
+      day: "numeric",
+      timeZone: "UTC",
+    });
+  }
+  return d.getHours().toString().padStart(2, "0") + ":00";
 }