fix(web): usage chart tooltip at the pointer's lower-right + cache hit rate in the Cache Read bubble (#37)

Co-authored-by: Alice <alice@prismshadow.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

packages/web/src/features/traces/trace-file-view.tsx

@@ -34,6 +34,7 @@ import * as api from "../../api/endpoints";
 import { ApiError } from "../../api/client";
 import { S } from "../../lib/strings";
 import {
+  cacheHitRate,
   computeTps,
   formatMoney,
   formatPercent,
@@ -103,11 +104,8 @@ function SummaryRow({ label, value }: { label: string; value: string }) {
 /** This round's input = cache hit (cacheRead) + cache miss (cacheWrite). */
 const inputOf = (b: Buckets): number => b.cacheRead + b.cacheWrite;
 
-/** Cache hit rate = cache hit ÷ this round's input; undefined when input is 0 → null (shown as `—`). */
+/** Cache hit rate of this round's input: the shared formula (lib/format.ts cacheHitRate, also used by the Cost center's bubble); input 0 → null (shown as `—`). */
-const hitRateOf = (b: Buckets): number | null => {
+const hitRateOf = (b: Buckets): number | null => cacheHitRate(b.cacheRead, b.cacheWrite);
-  const input = inputOf(b);
-  return input > 0 ? b.cacheRead / input : null;
-};
 
 /** Shared style for stat rows (icon + value, tabular figures). */
 const CHIP_CLASS =

packages/web/src/features/usage/chart-geom.ts

@@ -7,7 +7,8 @@
  * horizontal layout (fixed 25px bar width, spacing ≥ bar width) is computed
  * by tokenBarLayout, and per-segment geometry (including per-segment hit
  * bands) is produced by barSegments; there's also pie-slice geometry (each
- * Agent's call count) and success-rate normalization. See chart-svg.tsx for the render skeleton.
+ * Agent's call count), success-rate normalization, and hover-bubble
+ * placement (pointer lower-right, flipping at the edges). See chart-svg.tsx for the render skeleton.
  *
  * **Canvas width = the container's measured pixel width (1 canvas unit = 1
  * CSS pixel)**: the SVG no longer stretches/scales via a fixed viewBox —
@@ -112,6 +113,46 @@ export function successRate(completed: number, total: number): number {
   return total > 0 ? completed / total : 1;
 }
 
+// —— Hover bubble placement (shared by both daily charts) ——
+
+/** Gap between the pointer and the bubble's near corner: close enough to read as attached, far enough that the bubble never sits under the pointer. */
+export const BUBBLE_OFFSET = 12;
+
+/** The window the bubble must stay inside, in canvas coordinates: the scroll container's currently visible region (left/right move with horizontal scroll; the top is always 0). */
+export interface BubbleView {
+  left: number;
+  right: number;
+  bottom: number;
+}
+
+/**
+ * Hover bubble placement: the preferred spot is the pointer's lower-right
+ * (pointer + BUBBLE_OFFSET on both axes). Near an edge it **flips** to the
+ * pointer's other side (right edge → lower-left, bottom edge → upper-right,
+ * corner → upper-left): flipping keeps the bubble out from under the
+ * pointer, where pure clamping would slide it back over the hovered mark.
+ * The final clamp only guards the degenerate case (a window narrower than
+ * the bubble on both sides of the pointer): the bubble then covers the
+ * pointer, and a window narrower than the bubble itself still clips on the
+ * right — unreachable at real card widths; the clamp just keeps the failure graceful.
+ */
+export function bubblePosition(
+  px: number,
+  py: number,
+  bubbleW: number,
+  bubbleH: number,
+  view: BubbleView,
+): { left: number; top: number } {
+  let left = px + BUBBLE_OFFSET;
+  if (left + bubbleW > view.right) left = px - BUBBLE_OFFSET - bubbleW;
+  let top = py + BUBBLE_OFFSET;
+  if (top + bubbleH > view.bottom) top = py - BUBBLE_OFFSET - bubbleH;
+  return {
+    left: Math.max(view.left, Math.min(left, view.right - bubbleW)),
+    top: Math.max(0, Math.min(top, view.bottom - bubbleH)),
+  };
+}
+
 // —— Daily Token: bar + three-segment stack ——
 
 /**

packages/web/src/features/usage/chart-svg.tsx

@@ -3,7 +3,8 @@
  * original TrendChart, reused by both the daily Token stacked bar and the
  * daily cost line): 4 horizontal grid lines + y-axis ticks, x-axis dates, a
  * hover vertical indicator line + a transparent hit area + a value bubble
- * that follows the cursor. "Data marks" (line / area / bars) are drawn by
+ * that follows the cursor at its lower-right (flipping to the other side of
+ * the pointer near the edges, see chart-geom's bubblePosition). "Data marks" (line / area / bars) are drawn by
  * the caller as children in the same x()/y() coordinate system;
  * see chart-geom.ts for the coordinate math.
  *
@@ -23,11 +24,23 @@
  * already indicates the x position — an extra vertical line would just be
  * noise, so the bar chart passes hoverLine={false} to turn it off.
  */
-import { useLayoutEffect, useRef, useState, type ReactNode, type RefObject } from "react";
+import {
-import { CHART_H, PAD_L, PAD_R, PAD_T, sparseLabelIdx, type ChartGeom } from "./chart-geom";
+  useEffect,
-
+  useLayoutEffect,
-/** Upper bound on bubble width: clamps the bubble back inside the canvas near the right edge, so it doesn't spuriously trigger extra horizontal scroll. */
+  useRef,
-const BUBBLE_W = 160;
+  useState,
+  type ReactNode,
+  type RefObject,
+} from "react";
+import {
+  bubblePosition,
+  CHART_H,
+  PAD_L,
+  PAD_R,
+  PAD_T,
+  sparseLabelIdx,
+  type ChartGeom,
+} from "./chart-geom";
 
 /**
  * Measure the available width inside the chart card (CSS pixels, rounded
@@ -103,18 +116,77 @@ export function ChartFrame({
     el.scrollLeft = el.scrollWidth - el.clientWidth;
   }, [scrollToEnd, w, dates.length]);
 
+  // The bubble follows the pointer **imperatively** (lower-right, flipping
+  // at the edges — see chart-geom's bubblePosition): mousemove only records
+  // the client position and schedules a single rAF, whose callback batches
+  // the layout reads (svg origin, the scroll container's visible window,
+  // the bubble's real size) and then writes style.left/top directly. No
+  // React state is involved, so a 60–120Hz pointer neither re-renders the
+  // few-hundred-element svg nor forces multiple sync layouts per event.
+  // The bubble mounts hidden and is revealed by its first placement (on
+  // entry, mouseenter fires before the first mousemove has sampled the pointer).
+  const svgRef = useRef<SVGSVGElement>(null);
+  const bubbleRef = useRef<HTMLDivElement>(null);
+  const mouseRef = useRef<{ cx: number; cy: number } | null>(null);
+  const rafRef = useRef(0);
+  const placeBubble = () => {
+    const el = bubbleRef.current;
+    const svg = svgRef.current;
+    const sc = scrollRef.current;
+    const m = mouseRef.current;
+    if (!el || !svg || !sc || !m) return;
+    const r = svg.getBoundingClientRect();
+    // Pointer in canvas coordinates (the svg's top-left is the content
+    // origin; bubble and content scroll together, so content pixels are
+    // enough), bounded by the *visible* window rather than the full canvas —
+    // that is what keeps the bubble on screen when the Token bar canvas is scrolled.
+    const pos = bubblePosition(m.cx - r.left, m.cy - r.top, el.offsetWidth, el.offsetHeight, {
+      left: sc.scrollLeft,
+      right: sc.scrollLeft + sc.clientWidth,
+      bottom: CHART_H,
+    });
+    el.style.left = `${pos.left}px`;
+    el.style.top = `${pos.top}px`;
+    el.style.visibility = "visible";
+  };
+  const schedulePlace = () => {
+    if (rafRef.current) return;
+    rafRef.current = requestAnimationFrame(() => {
+      rafRef.current = 0;
+      placeBubble();
+    });
+  };
+  useEffect(() => () => cancelAnimationFrame(rafRef.current), []);
+
+  // Re-place synchronously (before paint) when the bubble's content changes:
+  // its size changes as hover moves between days/buckets, which can change
+  // the flip decision. Keyed on the hover anchor and the bubble renderer's
+  // identity (the parent re-creates the callback whenever its own hover
+  // state — e.g. the bar chart's segment key — changes) instead of running on every render.
+  useLayoutEffect(placeBubble, [bubble, hover]);
+
   return (
     // Horizontal scroll when the canvas is wider than the container (bar
     // width has a pixel floor, so 30 days won't fit in a half-width panel);
-    // the bubble is this container's absolutely-positioned child element and scrolls along with the content, so anchoring it to the column by pixels is enough.
+    // the bubble is this container's absolutely-positioned child element and scrolls along with the content, so anchoring it to the pointer in content pixels is enough.
     <div ref={scrollRef} className="relative overflow-x-auto">
       <svg
+        ref={svgRef}
         viewBox={`0 0 ${w} ${CHART_H}`}
         width={w}
         height={CHART_H}
         className="text-gray-600 dark:text-gray-400"
         role="img"
-        onMouseLeave={() => onHover(null)}
+        onMouseLeave={() => {
+          onHover(null);
+          // Drop the pointer sample: on re-entry the bubble stays hidden until the fresh position is known, rather than flashing at the stale one.
+          mouseRef.current = null;
+        }}
+        // Only record the client position and coalesce into one rAF: all layout reads happen inside the rAF callback (see placeBubble).
+        onMouseMove={(e) => {
+          mouseRef.current = { cx: e.clientX, cy: e.clientY };
+          schedulePlace();
+        }}
       >
         {/* Grid lines and y-axis ticks (recessive gray) */}
         {gridLevels.map((v, i) => (
@@ -190,11 +262,15 @@ export function ChartFrame({
 
       {bubble && hover !== null && dates[hover] && (
         <div
-          className="pointer-events-none absolute top-0 rounded border border-gray-200 bg-white px-2 py-1 text-xs shadow-sm dark:border-gray-700 dark:bg-gray-900"
+          ref={bubbleRef}
-          // Anchored near that column's left edge, clamped back inside the
+          className="pointer-events-none absolute rounded border border-gray-200 bg-white px-2 py-1 text-xs whitespace-nowrap shadow-sm dark:border-gray-700 dark:bg-gray-900"
-          // canvas (1 unit = 1 pixel, positioned directly in pixels; the
+          // Mounted hidden at the origin; placeBubble moves it to the
-          // left edge can't be negative, since the scroll container would clip off the part that sticks out).
+          // pointer's lower-right (flipping near the right/bottom edges, see
-          style={{ left: `${Math.max(0, Math.min(x(hover) - 30, w - BUBBLE_W))}px` }}
+          // bubblePosition) and reveals it. The declared style below never
+          // changes between renders, so React leaves the imperative
+          // left/top/visibility writes alone (nowrap keeps the measured
+          // width the true content width, independent of where the bubble lands).
+          style={{ left: 0, top: 0, visibility: "hidden" }}
         >
           {bubble(hover)}
         </div>

packages/web/src/features/usage/usage-charts.tsx

@@ -25,7 +25,7 @@ import type {
 } from "@prismshadow/penguin-server/api";
 import { catalogEntryFor, providerInfo } from "@prismshadow/penguin-core/model-catalog";
 import { S } from "../../lib/strings";
-import { humanizeTokens } from "../../lib/format";
+import { cacheHitRate, formatPercent, humanizeTokens } from "../../lib/format";
 import { TOKEN_COLORS } from "../../lib/token-colors";
 import { categoryColor } from "../../lib/category-colors";
 import {
@@ -254,7 +254,8 @@ interface SegHover {
  * be un-hoverable; widening the bar doesn't help the vertical dimension
  * either). Hitting a segment highlights only that segment and fades out
  * everything else; the bubble reports only that segment's date/bucket
- * name/Token count. When legend is passed in (legend hover), it highlights all segments of the matching bucket.
+ * name/Token count (the cacheRead segment adds that day's cache hit rate,
+ * cacheRead / (cacheRead + cacheWrite)). When legend is passed in (legend hover), it highlights all segments of the matching bucket.
  * No hover vertical line is drawn (hoverLine={false}): the bar itself already indicates the x position.
  */
 export function TokenBarChart({
@@ -304,12 +305,22 @@ export function TokenBarChart({
             const p = trend[i]!;
             const key = hover?.key;
             if (!key) return null;
+            // The cacheRead bubble additionally reports that day's cache hit
+            // rate, via the formula/format/label shared with the Trace page
+            // (lib/format.ts cacheHitRate + formatPercent, S.traces.hitRate),
+            // so the metric reads identically everywhere; null (denominator 0) omits the line instead of showing 0/0.
+            const hitRate = key === "cacheRead" ? cacheHitRate(p.cacheRead, p.cacheWrite) : null;
             return (
               <>
                 <p className="text-gray-400">{p.date}</p>
                 <p className="font-mono">
                   {bucketLabel(key)} {humanizeTokens(p[key])}
                 </p>
+                {hitRate !== null && (
+                  <p className="font-mono">
+                    {S.traces.hitRate} {formatPercent(hitRate)}
+                  </p>
+                )}
               </>
             );
           }}

packages/web/src/lib/format.ts

@@ -72,11 +72,22 @@ export function formatTps(tps: number | null | undefined): string {
   return `${v} tok/s`;
 }
 
+/**
+ * Cache hit rate = cacheRead ÷ (cacheRead + cacheWrite): the share of cached
+ * input actually served from cache; null when the denominator is 0 (no cache
+ * activity — the rate is undefined, callers omit the stat or let
+ * formatPercent render `—`). The single formula shared by the Trace page's
+ * turn/global summaries and the Cost center's cacheRead bubble, so the two pages can never drift apart.
+ */
+export function cacheHitRate(cacheRead: number, cacheWrite: number): number | null {
+  const total = cacheRead + cacheWrite;
+  return total > 0 ? cacheRead / total : null;
+}
+
 /**
  * Ratio display (rounded to a whole percent): `0.714` → `71%`; null
  * (denominator is 0, undefined) or a non-finite value → `—`.
- * Used by the Trace page's **cache hit rate** (cache hits ÷ this turn's
+ * Used with cacheHitRate above (Trace page and the Cost center's cacheRead bubble) and similar cases.
- * input) and similar cases.
  */
 export function formatPercent(ratio: number | null | undefined): string {
   if (ratio == null || !Number.isFinite(ratio)) return "—";

packages/web/test/format.test.ts

@@ -3,6 +3,7 @@
  */
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 import {
+  cacheHitRate,
   computeTps,
   formatBytes,
   formatDateTime,
@@ -93,6 +94,26 @@ describe("formatPercent", () => {
   });
 });
 
+describe("cacheHitRate (shared by the Trace summaries and the Cost center's cacheRead bubble)", () => {
+  it("cacheRead ÷ (cacheRead + cacheWrite)", () => {
+    expect(cacheHitRate(50, 50)).toBe(0.5);
+    expect(cacheHitRate(75, 25)).toBe(0.75);
+    expect(cacheHitRate(100, 0)).toBe(1);
+    expect(cacheHitRate(0, 100)).toBe(0); // all writes, no hits: 0 is a real value, not the guard
+  });
+
+  it("renders via formatPercent as a whole percent", () => {
+    expect(formatPercent(cacheHitRate(1, 2))).toBe("33%"); // 33.3… rounds down
+    expect(formatPercent(cacheHitRate(2, 1))).toBe("67%"); // 66.6… rounds up
+    expect(formatPercent(cacheHitRate(999, 1))).toBe("100%"); // 99.9 rounds to 100% at whole-percent precision
+  });
+
+  it("denominator 0 (no cache activity) yields null: the bubble omits the line, formatPercent shows —", () => {
+    expect(cacheHitRate(0, 0)).toBeNull();
+    expect(formatPercent(cacheHitRate(0, 0))).toBe("—");
+  });
+});
+
 describe("formatBytes", () => {
   it("byte abbreviation", () => {
     expect(formatBytes(812)).toBe("812B");

packages/web/test/usage-charts.test.ts

@@ -2,8 +2,10 @@
  * Cost Center chart pure-function unit tests (chart-geom.ts): coordinate mapping, SVG path
  * assembly, Token bar chart horizontal layout (fixed 25px bar width / spacing ≥ bar width /
  * whether it scrolls horizontally), stacked-bar segment geometry and per-segment hit bands,
- * pie slice arcs, success rate. Component interaction isn't covered here (vitest runs in a
+ * pie slice arcs, success rate, hover-bubble placement (pointer lower-right, flipping at
- * node environment, no DOM).
+ * the edges; the cache hit rate shown in the cacheRead bubble is lib/format's shared
+ * cacheHitRate, tested in format.test.ts). Component interaction isn't covered here
+ * (vitest runs in a node environment, no DOM).
  *
  * Canvas width is "measured container pixels" (1 canvas unit = 1 CSS pixel), so each case
  * passes an explicit width; 640 was the original fixed canvas width, and reusing it as the
@@ -18,10 +20,13 @@ import {
   sparseLabelIdx,
   autoLabelIdx,
   successRate,
+  bubblePosition,
   tokenBarLayout,
   barSegments,
   pieSlices,
   BAR_W,
+  BUBBLE_OFFSET,
+  CHART_H,
   MIN_HIT_H,
   PAD_L,
   PAD_R,
@@ -250,3 +255,61 @@ describe("successRate", () => {
     expect(successRate(0, 0)).toBe(1);
   });
 });
+
+describe("bubblePosition", () => {
+  // A 640px canvas that fits its card, not scrolled: the visible window is the whole canvas.
+  const view = { left: 0, right: 640, bottom: CHART_H };
+  const BW = 160;
+  const BH = 48;
+
+  it("default: the bubble hangs at the pointer's lower-right, offset on both axes", () => {
+    expect(bubblePosition(100, 50, BW, BH, view)).toEqual({
+      left: 100 + BUBBLE_OFFSET,
+      top: 50 + BUBBLE_OFFSET,
+    });
+  });
+
+  it("right edge: flips to the pointer's lower-left (clamping would slide it back under the pointer)", () => {
+    const pos = bubblePosition(600, 50, BW, BH, view);
+    expect(pos).toEqual({ left: 600 - BUBBLE_OFFSET - BW, top: 50 + BUBBLE_OFFSET });
+    expect(pos.left + BW).toBeLessThanOrEqual(view.right); // fully inside the window
+    expect(pos.left + BW).toBeLessThanOrEqual(600 - BUBBLE_OFFSET); // and clear of the pointer
+  });
+
+  it("bottom edge: flips above the pointer", () => {
+    expect(bubblePosition(100, 190, BW, BH, view)).toEqual({
+      left: 100 + BUBBLE_OFFSET,
+      top: 190 - BUBBLE_OFFSET - BH,
+    });
+  });
+
+  it("bottom-right corner: flips on both axes to the pointer's upper-left", () => {
+    expect(bubblePosition(630, 195, BW, BH, view)).toEqual({
+      left: 630 - BUBBLE_OFFSET - BW,
+      top: 195 - BUBBLE_OFFSET - BH,
+    });
+  });
+
+  it("an exact fit against the edge does not flip", () => {
+    const px = view.right - BUBBLE_OFFSET - BW; // left + BW lands exactly on view.right
+    expect(bubblePosition(px, 50, BW, BH, view).left).toBe(px + BUBBLE_OFFSET);
+  });
+
+  it("scrolled Token bar canvas: flips against the *visible* window, not the full canvas", () => {
+    // 1554px canvas in a 495px card scrolled to the far right: visible [1059, 1554].
+    const v = { left: 1059, right: 1554, bottom: CHART_H };
+    // Mid-window: normal lower-right placement (canvas coordinates, not window-relative).
+    expect(bubblePosition(1100, 50, BW, BH, v)).toEqual({ left: 1112, top: 62 });
+    // Near the visible right edge: 1512+160 would clip at 1554 → flip left.
+    expect(bubblePosition(1500, 50, BW, BH, v).left).toBe(1500 - BUBBLE_OFFSET - BW);
+    // Near the visible left edge the lower-right placement already fits: no shove.
+    expect(bubblePosition(1065, 50, BW, BH, v).left).toBe(1065 + BUBBLE_OFFSET);
+  });
+
+  it("degenerate guard: when neither side of the pointer fits, clamp to the window (covering the pointer is then unavoidable)", () => {
+    const v = { left: 0, right: 200, bottom: CHART_H };
+    const pos = bubblePosition(100, 100, 180, BH, v); // wider than either side of the pointer, still narrower than the window
+    expect(pos.left).toBe(0); // flip target would be negative → clamped to the window's left edge
+    expect(pos.left + 180).toBeLessThanOrEqual(v.right); // a bubble wider than the whole window would still clip on the right — unreachable at real card widths
+  });
+});