fix: keep line chart time labels from overlapping

src/ui/components/charts/LineChart.tsx

@@ -1,6 +1,7 @@
 import { useRef, useState, useEffect, useCallback } from "react";
 import { cn } from "@/lib/utils";
 import { parseServerTime } from "@/lib/server-time";
+import { formatTimestamp, pickXTicks } from "./x-ticks";
 
 export interface LineChartSeries {
   key: string;
@@ -24,26 +25,6 @@ const X_LABEL_H = 20;
 const TOP_PAD = 6;
 const TICK_COUNT = 5;
 
-function formatTimestamp(ts: string, rangeMs: number): string {
-  const d = parseServerTime(ts);
-  // A live chart covers a minute or two, so minutes alone would repeat.
-  if (rangeMs <= 10 * 60 * 1000) {
-    return d.toLocaleTimeString([], {
-      hour: "2-digit",
-      minute: "2-digit",
-      second: "2-digit",
-    });
-  }
-  if (rangeMs <= 25 * 60 * 60 * 1000) {
-    return d.toLocaleTimeString([], { hour: "2-digit", minute: "2-digit" });
-  }
-  return (
-    d.toLocaleDateString([], { month: "short", day: "numeric" }) +
-    " " +
-    d.toLocaleTimeString([], { hour: "2-digit", minute: "2-digit" })
-  );
-}
-
 function valToY(v: number, drawH: number, domain: [number, number]): number {
   const [lo, hi] = domain;
   const range = hi === lo ? 1 : hi - lo;
@@ -119,18 +100,7 @@ export function LineChart({
     return Array.from({ length: TICK_COUNT }, (_, i) => lo + i * step);
   })();
 
-  const xTickIndices: number[] = [];
-  if (timestamps.length > 1 && chartW > 0) {
-    const n = timestamps.length;
-    // Long labels ("Jun 28 14:30") are ~80px; short ("14:30") are ~40px.
-    // Use at most 3 ticks for long labels, 5 for short, so they never crowd.
-    const longLabel = rangeMs > 25 * 60 * 60 * 1000;
-    const maxTicks = longLabel ? 3 : 5;
-    const count = Math.min(maxTicks, n);
-    for (let k = 0; k < count; k++) {
-      xTickIndices.push(Math.round((k / (count - 1)) * (n - 1)));
-    }
-  }
+  const xTickIndices = pickXTicks(timestamps, rangeMs, chartW);
 
   const handleMouseMove = useCallback(
     (e: React.MouseEvent<SVGElement>) => {

src/ui/components/charts/x-ticks.ts

@@ -0,0 +1,74 @@
+import { parseServerTime } from "@/lib/server-time";
+
+export function formatTimestamp(ts: string, rangeMs: number): string {
+  const d = parseServerTime(ts);
+  // A live chart covers a minute or two, so minutes alone would repeat.
+  if (rangeMs <= 10 * 60 * 1000) {
+    return d.toLocaleTimeString([], {
+      hour: "2-digit",
+      minute: "2-digit",
+      second: "2-digit",
+    });
+  }
+  if (rangeMs <= 25 * 60 * 60 * 1000) {
+    return d.toLocaleTimeString([], { hour: "2-digit", minute: "2-digit" });
+  }
+  return (
+    d.toLocaleDateString([], { month: "short", day: "numeric" }) +
+    " " +
+    d.toLocaleTimeString([], { hour: "2-digit", minute: "2-digit" })
+  );
+}
+
+// Rough width of a 10px label, used to keep x labels apart.
+const CHAR_W = 6.2;
+const LABEL_GAP = 12;
+
+/**
+ * Picks which timestamps get an x label. The count follows the chart width and
+ * label length, and a label that would touch its neighbour is dropped.
+ */
+export function pickXTicks(
+  timestamps: string[],
+  rangeMs: number,
+  chartW: number,
+): number[] {
+  const n = timestamps.length;
+  if (n < 2 || chartW <= 0) return [];
+  const labelW = (i: number) =>
+    formatTimestamp(timestamps[i], rangeMs).length * CHAR_W;
+  const widest = Math.max(labelW(0), labelW(n - 1));
+  const fit = Math.floor((chartW + LABEL_GAP) / (widest + LABEL_GAP));
+  const count = Math.max(1, Math.min(5, n, fit));
+  const candidates =
+    count === 1
+      ? [n - 1]
+      : Array.from({ length: count }, (_, k) =>
+          Math.round((k / (count - 1)) * (n - 1)),
+        );
+
+  // Box of a label as drawn: first is start anchored, last end, others middle.
+  const box = (i: number): [number, number] => {
+    const x = (i / (n - 1)) * chartW;
+    const w = labelW(i);
+    if (i === 0) return [x, x + w];
+    if (i === n - 1) return [x - w, x];
+    return [x - w / 2, x + w / 2];
+  };
+
+  const clashes = (a: number | undefined, b: number) =>
+    a !== undefined && box(a)[1] + LABEL_GAP > box(b)[0];
+  const placed: number[] = [];
+  for (const i of [...new Set(candidates)]) {
+    // The newest time stays visible on a live chart, older labels make room.
+    if (i === n - 1) {
+      while (placed.length && clashes(placed[placed.length - 1], i)) {
+        placed.pop();
+      }
+    } else if (clashes(placed[placed.length - 1], i)) {
+      continue;
+    }
+    placed.push(i);
+  }
+  return placed;
+}

src/ui/tests/components/charts/line-chart-ticks.test.ts

@@ -0,0 +1,46 @@
+import { describe, it, expect } from "vitest";
+import { pickXTicks } from "../../../components/charts/x-ticks";
+
+const live = (n: number) =>
+  Array.from({ length: n }, (_, i) =>
+    new Date(Date.UTC(2026, 10, 4, 15, 37, i * 2)).toISOString(),
+  );
+
+describe("pickXTicks", () => {
+  it("returns nothing without two points or width", () => {
+    expect(pickXTicks(live(1), 0, 300)).toEqual([]);
+    expect(pickXTicks(live(5), 8000, 0)).toEqual([]);
+  });
+
+  it("uses up to five labels on a wide chart", () => {
+    const ts = live(60);
+    expect(pickXTicks(ts, 118_000, 900)).toHaveLength(5);
+  });
+
+  it("fits fewer labels on a narrow chart and keeps the newest", () => {
+    const ts = live(60);
+    const ticks = pickXTicks(ts, 118_000, 220);
+    expect(ticks.length).toBeLessThan(5);
+    expect(ticks[ticks.length - 1]).toBe(59);
+  });
+
+  it("never lets labels touch", () => {
+    for (const w of [120, 180, 220, 260, 300, 400, 600]) {
+      for (const n of [2, 3, 4, 5, 8, 60]) {
+        const ts = live(n);
+        const range = (n - 1) * 2000;
+        const ticks = pickXTicks(ts, range, w);
+        const labelW = 11 * 6.2;
+        const boxes = ticks.map((i) => {
+          const x = (i / (n - 1)) * w;
+          if (i === 0) return [x, x + labelW];
+          if (i === n - 1) return [x - labelW, x];
+          return [x - labelW / 2, x + labelW / 2];
+        });
+        for (let k = 1; k < boxes.length; k++) {
+          expect(boxes[k][0]).toBeGreaterThanOrEqual(boxes[k - 1][1]);
+        }
+      }
+    }
+  });
+});