From ba8596be961d6495dde98cb692e19a27d5583ab3 Mon Sep 17 00:00:00 2001
From: andig <andi@evcc.io>
Date: Sun, 16 Aug 2026 19:35:44 +0200
Subject: [PATCH] Optimizer chart: day boundaries with weekday at 00:00
 (#32894)

---
 assets/js/components/Optimize/ChargeChart.vue |  2 +-
 assets/js/components/Optimize/chart.test.ts   | 35 +++++++++++++++++++
 assets/js/components/Optimize/chart.ts        | 22 +++++++++---
 3 files changed, 54 insertions(+), 5 deletions(-)
 create mode 100644 assets/js/components/Optimize/chart.test.ts

diff --git a/assets/js/components/Optimize/ChargeChart.vue b/assets/js/components/Optimize/ChargeChart.vue
index 1b89cb910..ad03867d9 100644
--- a/assets/js/components/Optimize/ChargeChart.vue
+++ b/assets/js/components/Optimize/ChargeChart.vue
@@ -146,7 +146,7 @@ export default defineComponent({
 					...tooltipStyle(colors.text || ""),
 					formatter: this.tooltipFormatter,
 				},
-				xAxis: slotXAxis(this.times),
+				xAxis: slotXAxis(this.times, this.weekdayShort),
 				yAxis: forecastYAxis({
 					min: undefined,
 					position: "right",
diff --git a/assets/js/components/Optimize/chart.test.ts b/assets/js/components/Optimize/chart.test.ts
new file mode 100644
index 000000000..cca07af43
--- /dev/null
+++ b/assets/js/components/Optimize/chart.test.ts
@@ -0,0 +1,35 @@
+import { describe, expect, test } from "vite-plus/test";
+import { isMidnight, slotXAxis } from "./chart";
+
+// 15min slots starting at the given local hour
+function times(startHour: number, count: number): number[] {
+  const start = new Date(2025, 0, 1, startHour).getTime();
+  return Array.from({ length: count }, (_, i) => start + i * 15 * 60 * 1000);
+}
+
+describe("slotXAxis", () => {
+  const weekdayShort = () => "Thu";
+  // 20:00 to 08:00, midnight at index 16
+  const slots = times(20, 48);
+  const axis = slotXAxis(slots, weekdayShort);
+  const label = (i: number) => axis.axisLabel.formatter(String(slots[i]));
+
+  test("labels full hours only", () => {
+    expect(label(0)).toBe("20"); // 20:00
+    expect(label(1)).toBe(""); // 20:15
+    expect(label(4)).toBe(""); // 21:00, not a step hour
+    expect(label(32)).toBe("4"); // 04:00
+  });
+
+  test("shows the weekday at midnight", () => {
+    expect(label(16)).toBe("0\nThu");
+  });
+
+  test("draws a split line at day boundaries only", () => {
+    expect(isMidnight(slots[16])).toBe(true);
+    expect(axis.splitLine.interval(16)).toBe(true);
+    expect(axis.splitLine.interval(15)).toBe(false);
+    // no line at the axis start, even if the first slot is midnight
+    expect(slotXAxis(times(0, 4), weekdayShort).splitLine.interval(0)).toBe(false);
+  });
+});
diff --git a/assets/js/components/Optimize/chart.ts b/assets/js/components/Optimize/chart.ts
index 9ddf6f2ff..2aa026c4b 100644
--- a/assets/js/components/Optimize/chart.ts
+++ b/assets/js/components/Optimize/chart.ts
@@ -1,4 +1,5 @@
 import { xAxisLabelStyle } from "../Forecast/echarts";
+import colors from "@/colors";
 import type { BatteryDetail } from "@/types/evcc";
 
 // loadpoint part of a vehicle entry title: "Carport (blue e-Golf)" → "Carport"
@@ -19,22 +20,35 @@ export function slotTimes(timestamp: string, dt: number[]): number[] {
   return res;
 }
 
-// category x axis over slot times, labels at full hours every 4h (6h on mobile)
-export function slotXAxis(times: number[]) {
+export function isMidnight(time?: number): boolean {
+  if (time === undefined) return false;
+  const d = new Date(time);
+  return d.getHours() === 0 && d.getMinutes() === 0;
+}
+
+// category x axis over slot times, labels at full hours every 4h (6h on mobile),
+// day boundaries as split line with the weekday shown at 00:00
+export function slotXAxis(times: number[], weekdayShort: (d: Date) => string) {
   const step = window.innerWidth < 576 ? 6 : 4;
   return {
     type: "category",
     data: times.map(String),
     axisLine: { show: false },
     axisTick: { show: false },
-    splitLine: { show: false },
+    splitLine: {
+      show: true,
+      // split lines sit at the left edge of their slot
+      interval: (index: number) => index > 0 && isMidnight(times[index]),
+      lineStyle: { color: colors.muted || "", type: "solid" },
+    },
     axisLabel: {
       ...xAxisLabelStyle(),
       interval: 0,
       formatter: (value: string) => {
         const d = new Date(Number(value));
         if (d.getMinutes() !== 0 || d.getHours() % step !== 0) return "";
-        return String(d.getHours());
+        const label = String(d.getHours());
+        return d.getHours() === 0 ? `${label}\n${weekdayShort(d)}` : label;
       },
     },
   };