diff --git a/components/charts/utils/chart-data-transformers.test.ts b/components/charts/utils/chart-data-transformers.test.ts
new file mode 100644
index 0000000..a76ee95
--- /dev/null
+++ b/components/charts/utils/chart-data-transformers.test.ts
@@ -0,0 +1,82 @@
+import { describe, expect, it } from 'bun:test';
+import { mergeDataSeries } from './chart-data-transformers';
+
+type PricePoint = {
+  date: string;
+  price: number;
+};
+
+describe('mergeDataSeries', () => {
+  it('normalizes intraday timestamps onto the same trading day', () => {
+    const merged = mergeDataSeries<PricePoint>([
+      {
+        id: 'stock',
+        data: [
+          { date: '2026-03-10T21:00:00.000Z', price: 100 },
+          { date: '2026-03-11T21:00:00.000Z', price: 101 }
+        ]
+      },
+      {
+        id: 'sp500',
+        data: [
+          { date: '2026-03-10', price: 5000 },
+          { date: '2026-03-11', price: 5050 }
+        ]
+      }
+    ]);
+
+    expect(merged).toEqual([
+      { date: '2026-03-10', stock: 100, sp500: 5000 },
+      { date: '2026-03-11', stock: 101, sp500: 5050 }
+    ]);
+  });
+
+  it('fills a comparison series from the nearest prior trading day when dates are missing', () => {
+    const merged = mergeDataSeries<PricePoint>([
+      {
+        id: 'stock',
+        data: [
+          { date: '2026-03-10', price: 100 },
+          { date: '2026-03-11', price: 103 },
+          { date: '2026-03-12', price: 104 }
+        ]
+      },
+      {
+        id: 'sp500',
+        data: [
+          { date: '2026-03-10', price: 5000 },
+          { date: '2026-03-12', price: 5060 }
+        ]
+      }
+    ]);
+
+    expect(merged).toEqual([
+      { date: '2026-03-10', stock: 100, sp500: 5000 },
+      { date: '2026-03-11', stock: 103, sp500: 5000 },
+      { date: '2026-03-12', stock: 104, sp500: 5060 }
+    ]);
+  });
+
+  it('does not backfill dates before a series first appears', () => {
+    const merged = mergeDataSeries<PricePoint>([
+      {
+        id: 'stock',
+        data: [
+          { date: '2026-03-10', price: 100 },
+          { date: '2026-03-11', price: 103 }
+        ]
+      },
+      {
+        id: 'sp500',
+        data: [
+          { date: '2026-03-11', price: 5000 }
+        ]
+      }
+    ]);
+
+    expect(merged).toEqual([
+      { date: '2026-03-10', stock: 100 },
+      { date: '2026-03-11', stock: 103, sp500: 5000 }
+    ]);
+  });
+});
diff --git a/components/charts/utils/chart-data-transformers.ts b/components/charts/utils/chart-data-transformers.ts
index 3c060af..f306ccd 100644
--- a/components/charts/utils/chart-data-transformers.ts
+++ b/components/charts/utils/chart-data-transformers.ts
@@ -1,6 +1,15 @@
 import { subWeeks, subMonths, subYears } from 'date-fns';
 import type { TimeRange, ChartDataPoint } from '@/lib/types';
 
+function toTradingDayKey(value: string): string {
+  const parsed = Date.parse(value);
+  if (!Number.isFinite(parsed)) {
+    return value;
+  }
+
+  return new Date(parsed).toISOString().slice(0, 10);
+}
+
 /**
  * Filter chart data by time range
  */
@@ -128,30 +137,55 @@ export function calculateVolumeMax<T extends ChartDataPoint>(data: T[]): number
  */
 export function mergeDataSeries<T extends ChartDataPoint>(
   seriesArray: Array<{ id: string; data: T[] }>
-): Array<T & Record<string, number>> {
+): Array<{ date: string } & Record<string, string | number>> {
   if (!seriesArray || seriesArray.length === 0) return [];
 
-  // Create map indexed by date
-  const dateMap = new Map<string, T & Record<string, number>>();
+  // Create map indexed by normalized trading day.
+  const dateMap = new Map<string, { date: string } & Record<string, string | number>>();
+  const seriesPointMaps = seriesArray.map((series) => {
+    const pointMap = new Map<string, number>();
 
-  seriesArray.forEach(series => {
     series.data.forEach(point => {
-      const date = point.date;
-      const existing = dateMap.get(date) || { date } as T & Record<string, number>;
+      const date = toTradingDayKey(point.date);
 
-      // Add value for this series
       if (isOHLCVData(point)) {
-        existing[series.id] = point.close;
+        pointMap.set(date, point.close);
       } else if (isPriceData(point)) {
-        existing[series.id] = point.price;
+        pointMap.set(date, point.price);
+      }
+    });
+
+    pointMap.forEach((_value, date) => {
+      const existing = dateMap.get(date) || { date } as { date: string } & Record<string, string | number>;
+      dateMap.set(date, existing);
+    });
+
+    return {
+      id: series.id,
+      pointMap
+    };
+  });
+
+  const mergedDates = Array.from(dateMap.keys()).sort((a, b) =>
+    new Date(a).getTime() - new Date(b).getTime()
+  );
+
+  // Fill forward each series to keep benchmark/comparison lines aligned on trading days.
+  seriesPointMaps.forEach(({ id, pointMap }) => {
+    let lastKnownValue: number | null = null;
+
+    mergedDates.forEach((date) => {
+      const currentValue = pointMap.get(date);
+      if (typeof currentValue === 'number') {
+        lastKnownValue = currentValue;
       }
 
-      dateMap.set(date, existing);
+      const existing = dateMap.get(date);
+      if (existing && lastKnownValue !== null) {
+        existing[id] = lastKnownValue;
+      }
     });
   });
 
-  // Convert to array and sort by date
-  return Array.from(dateMap.values()).sort((a, b) =>
-    new Date(a.date).getTime() - new Date(b.date).getTime()
-  );
+  return mergedDates.map((date) => dateMap.get(date)!);
 }