Stats: heatmap uses the theme accent with a continuous color ramp

Cells blend in OKLab from a neutral empty color to the theme's accent on a
square-root scale instead of seven fixed blue steps; the legend shows the
gradient.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/components/Charts.tsx

@@ -6,7 +6,23 @@ import { useEffect, useRef, useState, type ReactNode } from "react";
 import { useT } from "../i18n";
 
 export const SERIES = Array.from({ length: 8 }, (_, i) => `var(--series-${i + 1})`);
-const SEQ = Array.from({ length: 7 }, (_, i) => `var(--seq-${i})`);
+/**
+ * Sequential heat color from the theme: empty cells use the neutral --heat-empty,
+ * any value > 0 blends continuously (in OKLab, so lightness changes evenly) from a
+ * faint tint to the full theme accent. A square-root scale keeps rare small values
+ * visible next to a few large peaks.
+ */
+const HEAT_MIN = 22;
+function heatShare(v: number, max: number): number {
+  if (v <= 0 || max <= 0) return 0;
+  return HEAT_MIN + (100 - HEAT_MIN) * Math.sqrt(Math.min(v / max, 1));
+}
+function heatColor(share: number): string {
+  return share <= 0 ? "var(--heat-empty)" : `color-mix(in oklab, var(--heat-full) ${share.toFixed(1)}%, var(--heat-empty))`;
+}
+const HEAT_GRADIENT = `linear-gradient(in oklab to right, ${[0, 0.1, 0.25, 0.45, 0.7, 1]
+  .map((f) => `${heatColor(heatShare(f * f, 1))} ${f * 100}%`)
+  .join(", ")})`;
 
 function useWidth<T extends HTMLElement>() {
   const ref = useRef<T>(null);
@@ -173,7 +189,7 @@ export function StackedBars({
   );
 }
 
-/** Day × hour heatmap with a 7-step sequential ramp. cells[row*24+hour]; rows top to bottom. */
+/** Day × hour heatmap with a continuous sequential ramp in the theme accent. cells[row*24+hour]; rows top to bottom. */
 export function Heatmap({
   cells,
   rows,
@@ -194,7 +210,6 @@ export function Heatmap({
   const cell = Math.min(cellW, 26);
   const max = Math.max(...cells, 0);
   const order = rows.map((_, i) => i);
-  const bin = (v: number) => (v <= 0 || max === 0 ? 0 : Math.min(6, 1 + Math.floor((v / max) * 5.999)));
 
   return (
     <div className="chart" ref={ref} onMouseLeave={() => setTip(undefined)}>
@@ -224,7 +239,7 @@ export function Heatmap({
                 width={cellW - 2}
                 height={cell - 2}
                 rx={3}
-                fill={SEQ[bin(v)]}
+                style={{ fill: heatColor(heatShare(v, max)) }}
                 onMouseMove={(e) => {
                   const box = (e.currentTarget.ownerSVGElement as SVGSVGElement).getBoundingClientRect();
                   setTip({
@@ -246,12 +261,10 @@ export function Heatmap({
         )}
       </svg>
       <div className="legend" style={{ marginTop: 6 }}>
-        <i style={{ background: SEQ[0], width: 18 }} />
+        <i style={{ background: heatColor(0), width: 18 }} />
         <span>{t("stats.heatEmpty")}</span>
         <span style={{ marginLeft: 12 }}>{format(0)}</span>
-        {SEQ.slice(1).map((c, i) => (
-          <i key={i} style={{ background: c, width: 18 }} />
-        ))}
+        <i className="heat-scale" style={{ background: HEAT_GRADIENT }} />
         <span>{format(max)}</span>
       </div>
       {tip && (