fix(prs): fit time decay curve through subnet-reported multiplier

src/components/prs/PRTimeDecayChart.tsx

@@ -16,6 +16,7 @@ import {
   buildDecaySubline,
   type DecayParams,
   type DecayProjection,
+  fitCurveThroughPoint,
   resolveDecayParams,
 } from './prTimeDecayModel';
 
@@ -252,14 +253,18 @@ function buildChartOption(
   const fontFamily = echartsFontFamily(theme);
   const mutedHex = alpha(theme.palette.common.white, TEXT_OPACITY.tertiary);
   const marker = resolveNowMarker(projection);
-  const seriesData = injectNowPoint(buildDecayCurve(params), marker);
+  const curveParams =
+    projection.curveMismatch && marker
+      ? fitCurveThroughPoint(params, marker.day, marker.multiplier)
+      : params;
+  const seriesData = injectNowPoint(buildDecayCurve(curveParams), marker);
   return {
     ...echartsTransparentBackground(),
     grid: { left: 44, right: 16, top: 28, bottom: 36 },
     tooltip: buildTooltip(theme, projection.preDecayScore, mutedHex),
-    xAxis: buildXAxis(axis, fontFamily, params.lookbackDays),
+    xAxis: buildXAxis(axis, fontFamily, curveParams.lookbackDays),
     yAxis: buildYAxis(axis),
-    series: [buildSeries(theme, axis, params, seriesData, marker)],
+    series: [buildSeries(theme, axis, curveParams, seriesData, marker)],
   };
 }
 

src/components/prs/prTimeDecayModel.ts

@@ -104,6 +104,31 @@ export function resolveDecayParams(
   };
 }
 
+/**
+ * Re-fit the sigmoid midpoint so the curve passes through the subnet-reported
+ * (day, multiplier) point. Repos can override decay hyperparameters without
+ * the API publishing them (`/repos/{name}` only exposes emission shares), so
+ * when the reported multiplier contradicts the resolved params the fitted
+ * curve is the closest available approximation of the repo's real schedule.
+ */
+export function fitCurveThroughPoint(
+  params: DecayParams,
+  day: number,
+  multiplier: number,
+): DecayParams {
+  if (!(multiplier > params.floor) || !(multiplier < 1) || day <= 0) {
+    return params;
+  }
+  const midpoint = day - Math.log(1 / multiplier - 1) / params.steepness;
+  return {
+    ...params,
+    midpoint,
+    // A reported multiplier below 1 means decay already started, so the
+    // grace period cannot extend past the merge age.
+    graceHours: Math.min(params.graceHours, day * 24),
+  };
+}
+
 function decayAt(days: number, params: DecayParams): number {
   if (days <= params.graceHours / 24) return 1;
   const sigmoid =
@@ -214,6 +239,6 @@ export function buildDecaySubline(projection: DecayProjection): string {
   }
   const base = `${projection.daysSinceMerge.toFixed(1)} day(s) since merge`;
   return projection.curveMismatch
-    ? `${base} · showing subnet-reported multiplier (curve is approximate)`
+    ? `${base} · curve fitted to subnet-reported multiplier (repo overrides decay params)`
     : base;
 }