ActivityCharts: 10s rolling mean on cadence and power line charts (display only)

site/src/components/ActivityCharts.svelte

@@ -147,6 +147,20 @@
     };
   }
 
+  // ── Rolling mean (display only — raw data used for stats/histogram) ────
+  function rollingMean(vals: (number | null)[], halfWin: number): (number | null)[] {
+    return vals.map((_, i) => {
+      const lo = Math.max(0, i - halfWin);
+      const hi = Math.min(vals.length - 1, i + halfWin);
+      let sum = 0, count = 0;
+      for (let j = lo; j <= hi; j++) {
+        const v = vals[j];
+        if (v != null) { sum += v; count++; }
+      }
+      return count > 0 ? sum / count : null;
+    });
+  }
+
   // ── Reference lines (avg, P20, P80) — speed, cadence, power, hr ────────
   function refStats(tab: Tab, vals: number[]) {
     if (tab !== 'speed' && tab !== 'cadence' && tab !== 'power' && tab !== 'hr') return null;
@@ -214,12 +228,20 @@
     // control points and visual artifacts — use linear instead.
     const curve = xMode === 'distance' ? 'linear' : 'monotone-x';
 
+    // Smooth cadence and power for visual rendering only (±5 s centred window = 11 s).
+    // Raw data is still used for the histogram, reference-line stats, and tooltip.
+    const needsSmooth = activeTab === 'cadence' || activeTab === 'power';
+    const smoothed = needsSmooth
+      ? rollingMean(data.map(d => (d as any)[yKey] as number | null), 5)
+      : null;
+    const lineData = smoothed ? data.map((d, i) => ({ ...d, [yKey]: smoothed[i] })) : data;
+
     if (activeTab === 'cadence') {
-      marks.push(Plot.lineY(data, { x, y: yKey, stroke: color, strokeWidth: 1.5, curve }));
+      marks.push(Plot.lineY(lineData, { x, y: yKey, stroke: color, strokeWidth: 1.5, curve }));
     } else {
       marks.push(
-        Plot.areaY(data, { x, y: yKey, fill: color, fillOpacity: 0.15, curve }),
-        Plot.lineY(data, { x, y: yKey, stroke: color, strokeWidth: 1.5, curve }),
+        Plot.areaY(lineData, { x, y: yKey, fill: color, fillOpacity: 0.15, curve }),
+        Plot.lineY(lineData, { x, y: yKey, stroke: color, strokeWidth: 1.5, curve }),
       );
     }