diff --git a/src/pages/workouts/components/LineWithLineChart.ts b/src/pages/workouts/components/LineWithLineChart.ts
index 376e89d..8816a55 100644
--- a/src/pages/workouts/components/LineWithLineChart.ts
+++ b/src/pages/workouts/components/LineWithLineChart.ts
@@ -40,7 +40,7 @@ class LineWithLineController extends LineController {
     // pixel position must come from Chart.js itself, not from index * zoom.
     const meta = this.chart.getDatasetMeta(0);
     for (const i in xVertical) {
-      const idx = Number(i);
+      const idx = Number(xVertical[i]);
       const point = meta.data[idx] as { x?: number } | undefined;
       const x = point?.x ?? NaN;
       if (!Number.isFinite(x)) {