diff --git a/ui/src/components/test-history/TestHistoryChart.tsx b/ui/src/components/test-history/TestHistoryChart.tsx
index ddd79e22..a6c5c4ee 100644
--- a/ui/src/components/test-history/TestHistoryChart.tsx
+++ b/ui/src/components/test-history/TestHistoryChart.tsx
@@ -2,7 +2,13 @@ import { useMemo } from 'react'
 import ReactECharts from 'echarts-for-react'
 import { getClientChartColor } from '@/utils/client-colors'
 import { formatDuration } from '@/utils/format'
-import { formatHistoryValue, HISTORY_METRIC_LABELS, type HistoryMetric, type HistoryPoint } from '@/utils/testHistory'
+import {
+  findRunGaps,
+  formatHistoryValue,
+  HISTORY_METRIC_LABELS,
+  type HistoryMetric,
+  type HistoryPoint,
+} from '@/utils/testHistory'
 
 export type HistoryXAxis = 'time' | 'runCount'
 
@@ -14,12 +20,17 @@ interface TestHistoryChartProps {
   onRunClick: (runId: string) => void
 }
 
-// Above this many points per client, the chart hides the symbols and adds a
-// zoom slider.
+// Above this many points, a client line has small symbols and a thin line,
+// and the chart adds a zoom slider.
 const LARGE_SERIES = 30
+// Above this many points, a client line hides its symbols. They would cover
+// the line.
+const DENSE_SERIES = 100
 
 // [x, value, runId, image, runStart, runNumber]
 type ChartDatum = [number, number, string, string, number, number]
+// A point with no value breaks the line.
+type GapDatum = [number, null]
 
 function capitalizeFirst(str: string): string {
   return str ? str.charAt(0).toUpperCase() + str.slice(1) : str
@@ -35,21 +46,63 @@ export function TestHistoryChart({ pointsByClient, metric, xAxis, isDark, onRunC
     const maxSeriesLength = Math.max(0, ...[...pointsByClient.values()].map((p) => p.length))
     const isLarge = maxSeriesLength > LARGE_SERIES
 
-    const series = [...pointsByClient.entries()].map(([client, points]) => ({
-      name: capitalizeFirst(client),
-      type: 'line' as const,
-      // Run #1 is the most recent run of the client.
-      data: points.map((p, i): ChartDatum => {
+    const series = [...pointsByClient.entries()].flatMap(([client, points]) => {
+      const large = points.length > LARGE_SERIES
+      const dense = points.length > DENSE_SERIES
+      // On the time axis, the line breaks at a gap of days between runs.
+      const gaps = xAxis === 'time' ? findRunGaps(points.map((p) => p.runStart)) : new Set<number>()
+      const data: (ChartDatum | GapDatum)[] = []
+      // The points with a gap on both sides. A line with hidden symbols does
+      // not show them, so a scatter series does.
+      const isolated: ChartDatum[] = []
+
+      points.forEach((p, i) => {
+        // Run #1 is the most recent run of the client.
         const runNumber = points.length - i
-        return [xAxis === 'time' ? p.runStart * 1000 : runNumber, p.value, p.runId, p.image ?? '', p.runStart, runNumber]
-      }),
-      showSymbol: !isLarge,
-      symbolSize: isLarge ? 4 : 7,
-      lineStyle: { width: isLarge ? 1.5 : 2 },
-      itemStyle: { color: getClientChartColor(client) },
-      emphasis: { focus: 'series' as const },
-      cursor: 'pointer',
-    }))
+        const datum: ChartDatum = [
+          xAxis === 'time' ? p.runStart * 1000 : runNumber,
+          p.value,
+          p.runId,
+          p.image ?? '',
+          p.runStart,
+          runNumber,
+        ]
+
+        if (gaps.has(i)) data.push([datum[0], null])
+        data.push(datum)
+
+        if ((i === 0 || gaps.has(i)) && (i === points.length - 1 || gaps.has(i + 1))) isolated.push(datum)
+      })
+
+      const name = capitalizeFirst(client)
+      const color = getClientChartColor(client)
+      const line = {
+        name,
+        type: 'line' as const,
+        data,
+        showSymbol: !dense,
+        symbolSize: large ? 4 : 7,
+        lineStyle: { width: large ? 1.5 : 2 },
+        itemStyle: { color },
+        emphasis: { focus: 'series' as const },
+        cursor: 'pointer',
+      }
+      if (!dense || isolated.length === 0) return [line]
+
+      // The same name, so the legend shows and hides both series together.
+      return [
+        line,
+        {
+          name,
+          type: 'scatter' as const,
+          data: isolated,
+          symbolSize: 5,
+          itemStyle: { color },
+          emphasis: { focus: 'series' as const },
+          cursor: 'pointer',
+        },
+      ]
+    })
 
     const xAxisConfig =
       xAxis === 'time'
diff --git a/ui/src/pages/TestHistoryPage.tsx b/ui/src/pages/TestHistoryPage.tsx
index f0c2947a..8474c54f 100644
--- a/ui/src/pages/TestHistoryPage.tsx
+++ b/ui/src/pages/TestHistoryPage.tsx
@@ -171,7 +171,9 @@ export function TestHistoryPage() {
   const runsPerClient = RUNS_PER_CLIENT_OPTIONS.includes(search.n as (typeof RUNS_PER_CLIENT_OPTIONS)[number])
     ? search.n!
     : DEFAULT_RUNS_PER_CLIENT
-  const xAxis: HistoryXAxis = search.x === 'runCount' ? 'runCount' : 'time'
+  // Run # is the default: it spaces the runs evenly, so a gap of days between
+  // runs does not squeeze the recent runs.
+  const xAxis: HistoryXAxis = search.x === 'time' ? 'time' : 'runCount'
   const stepWindow = CHANGE_WINDOW_OPTIONS.includes(search.w as (typeof CHANGE_WINDOW_OPTIONS)[number])
     ? search.w!
     : DEFAULT_CHANGE_WINDOW
@@ -348,10 +350,10 @@ export function TestHistoryPage() {
                 ariaLabel="X axis"
                 value={xAxis}
                 options={[
-                  { value: 'time', label: 'Time' },
                   { value: 'runCount', label: 'Run #' },
+                  { value: 'time', label: 'Time' },
                 ]}
-                onChange={(value) => updateSearch({ x: value === 'time' ? undefined : value })}
+                onChange={(value) => updateSearch({ x: value === 'runCount' ? undefined : value })}
               />
             </div>
             <label className="flex items-center gap-2">
diff --git a/ui/src/utils/testHistory.test.ts b/ui/src/utils/testHistory.test.ts
index 8bbe81c4..12dceee3 100644
--- a/ui/src/utils/testHistory.test.ts
+++ b/ui/src/utils/testHistory.test.ts
@@ -1,7 +1,14 @@
 import { describe, expect, it } from 'vitest'
 
 import type { TestHistoryDuration } from '@/api/types'
-import { buildHistoryPoints, historyMetricValue, median, summarizeClient, type HistoryPoint } from './testHistory'
+import {
+  buildHistoryPoints,
+  findRunGaps,
+  historyMetricValue,
+  median,
+  summarizeClient,
+  type HistoryPoint,
+} from './testHistory'
 
 function duration(overrides: Partial<TestHistoryDuration>): TestHistoryDuration {
   return {
@@ -104,3 +111,27 @@ describe('summarizeClient', () => {
     expect(summarizeClient('geth', [])).toBeUndefined()
   })
 })
+
+describe('findRunGaps', () => {
+  const hour = 3600
+  const day = 24 * hour
+
+  it('breaks the line at a gap of days between runs every few hours', () => {
+    // One run, a gap of 5 days, then a run every 7 hours.
+    expect([...findRunGaps([0, 5 * day, 5 * day + 7 * hour, 5 * day + 14 * hour])]).toEqual([1])
+  })
+
+  it('keeps a steady series in one line', () => {
+    expect(findRunGaps([0, day, 2 * day, 3 * day, 4 * day]).size).toBe(0)
+  })
+
+  it('ignores a short gap between frequent runs', () => {
+    // Runs every 30 minutes, with one 4 hour gap: below the 6 hour minimum.
+    const starts = [0, 1800, 3600, 3600 + 4 * hour, 5400 + 4 * hour]
+    expect(findRunGaps(starts).size).toBe(0)
+  })
+
+  it('needs at least two intervals', () => {
+    expect(findRunGaps([0, 10 * day]).size).toBe(0)
+  })
+})
diff --git a/ui/src/utils/testHistory.ts b/ui/src/utils/testHistory.ts
index 60c543d8..638279ab 100644
--- a/ui/src/utils/testHistory.ts
+++ b/ui/src/utils/testHistory.ts
@@ -282,6 +282,26 @@ export function summarizeClient(
   }
 }
 
+// A gap between two runs breaks the chart line when it is GAP_FACTOR times the
+// median interval of the client, and at least MIN_GAP_SECONDS. A line across
+// a gap of days looks like a slow trend, but there are no runs in it.
+const GAP_FACTOR = 5
+const MIN_GAP_SECONDS = 6 * 3600
+
+// Returns the indexes of the points that start a new line after a gap. The
+// run starts must be in Unix seconds, oldest first.
+export function findRunGaps(runStarts: number[]): Set<number> {
+  const intervals = runStarts.slice(1).map((t, i) => t - runStarts[i])
+  const gaps = new Set<number>()
+  if (intervals.length < 2) return gaps
+
+  const limit = Math.max(GAP_FACTOR * median(intervals), MIN_GAP_SECONDS)
+  intervals.forEach((interval, i) => {
+    if (interval > limit) gaps.add(i + 1)
+  })
+  return gaps
+}
+
 // Formats a metric value for display.
 export function formatHistoryValue(value: number, metric: HistoryMetric): string {
   return metric === 'mgas' ? `${value.toFixed(2)} MGas/s` : formatDuration(Math.round(value))