diff --git a/frontend/src/components/AnalysisChart.tsx b/frontend/src/components/AnalysisChart.tsx
index 3e40f157..dcf1c775 100644
--- a/frontend/src/components/AnalysisChart.tsx
+++ b/frontend/src/components/AnalysisChart.tsx
@@ -1,13 +1,19 @@
 // © 2026 Joshua Benjamin Jewell. All rights reserved.
 // Licensed under the GNU Affero General Public License version 3 (AGPLv3).
 import Plotly from 'plotly.js-dist-min'
+import { stripHtmlTags } from '../utils/text'
 import { AddToReport } from './AddToReport'
 import { PlotlyChart } from './PlotlyChart'
 
+/**
+ * Read a Plotly title from `layout.title` or its `text` property and strip tags
+ * for the default report caption. A missing or empty title returns null; a
+ * title containing only tags returns an empty string. Whitespace is preserved.
+ */
 const figureTitle = (fig: unknown): string | null => {
   const t = (fig as { layout?: { title?: unknown } })?.layout?.title
   const text = typeof t === 'string' ? t : (t as { text?: string } | undefined)?.text
-  return text ? text.replace(/<[^>]+>/g, '') : null
+  return text ? stripHtmlTags(text) : null
 }
 
 // The chart as standalone SVG, at a fixed print width.
diff --git a/frontend/src/figure/charts.ts b/frontend/src/figure/charts.ts
index 29845147..dcb58c29 100644
--- a/frontend/src/figure/charts.ts
+++ b/frontend/src/figure/charts.ts
@@ -2,6 +2,7 @@
 // Licensed under the GNU Affero General Public License version 3 (AGPLv3).
 import { api } from '../api/client'
 import { extractAlphaPanel } from '../components/alphaMetrics'
+import { stripHtmlTags } from '../utils/text'
 import { textWidth } from './legend'
 import { PX_PER_PT, tickPt, titlePt, type AlphaMetricKey, type ChartSpec, type FigureDoc, type FigurePane } from './types'
 
@@ -101,7 +102,14 @@ function recolour(t: Record<string, unknown>, colours: Record<string, string>):
   return { ...t, marker: { ...marker, color: rgba(c, c[3]), ...(t.type === 'scatter' ? { line: outline } : {}) } }
 }
 
-/** The chart sized, typeset and titled for its pane. */
+/**
+ * Return a JSON-cloned copy of the chart, sized, typeset and titled for its pane.
+ * A null pane title keeps the chart's title; an empty or whitespace-only title
+ * hides it. Title markup is retained for rendering but stripped for measuring
+ * the font size needed to fit the available width.
+ * Requires a browser document for text measurement. JSON serialisation, parsing
+ * and text measurement errors propagate to the caller.
+ */
 export function paneFigure(raw: unknown, pane: FigurePane, doc: FigureDoc, opts: {
   showLegend: boolean
   widthPx: number
@@ -119,7 +127,7 @@ export function paneFigure(raw: unknown, pane: FigurePane, doc: FigureDoc, opts:
   // A title wider than its pane shrinks to fit, clear of the group letter.
   const titleRoom = opts.widthPx - 2 * doc.letters.sizePt * PX_PER_PT
   const titleSize = Math.min(titlePt(doc) * PX_PER_PT,
-    titlePt(doc) * PX_PER_PT * titleRoom / Math.max(1, textWidth(title.replace(/<[^>]+>/g, ''), titlePt(doc) * PX_PER_PT, family)))
+    titlePt(doc) * PX_PER_PT * titleRoom / Math.max(1, textWidth(stripHtmlTags(title), titlePt(doc) * PX_PER_PT, family)))
 
   for (const k of axisKeys(L, 'x').concat(axisKeys(L, 'y'))) {
     const ax = { ...(L[k] as Record<string, unknown>) }
diff --git a/frontend/src/utils/text.test.ts b/frontend/src/utils/text.test.ts
new file mode 100644
index 00000000..f5ed71b9
--- /dev/null
+++ b/frontend/src/utils/text.test.ts
@@ -0,0 +1,24 @@
+// © 2026 Joshua Benjamin Jewell. All rights reserved.
+// Licensed under the GNU Affero General Public License version 3 (AGPLv3).
+import { describe, expect, test } from 'bun:test'
+import { splitLines, stripHtmlTags } from './text'
+
+describe('stripHtmlTags', () => {
+  test('removes the markup Plotly allows in a title', () => {
+    expect(stripHtmlTags('CO<sub>2</sub> by <b>run</b>')).toBe('CO2 by run')
+  })
+
+  test('a tag hiding inside another does not survive', () => {
+    expect(stripHtmlTags('<scr<script>ipt>alert(1)</script>')).not.toMatch(/<[^>]+>/)
+  })
+
+  test('text without markup, including a bare >, is unchanged', () => {
+    expect(stripHtmlTags('p > 0.05 for all')).toBe('p > 0.05 for all')
+  })
+})
+
+describe('splitLines', () => {
+  test('one trimmed value per line, blanks dropped', () => {
+    expect(splitLines(' a\n\n b \nc')).toEqual(['a', 'b', 'c'])
+  })
+})
diff --git a/frontend/src/utils/text.ts b/frontend/src/utils/text.ts
index dc076655..fc28108a 100644
--- a/frontend/src/utils/text.ts
+++ b/frontend/src/utils/text.ts
@@ -9,3 +9,22 @@
 export function splitLines(text: string): string[] {
   return text.split('\n').map(v => v.trim()).filter(v => v.length > 0)
 }
+
+/**
+ * Strip the pseudo-HTML markup Plotly accepts in titles and axis labels (<b>,
+ * <br>, <sub> and friends), leaving the bare text for measuring or for a
+ * filename. A single pass is not enough: replacing "<scr<script>ipt>" once
+ * removes the inner tag and leaves the halves joined into another, so the
+ * strip repeats until the text stops changing.
+ * Entities are not decoded and removed tags add no spacing or line breaks.
+ * This is a text helper, not an HTML sanitiser.
+ */
+export function stripHtmlTags(text: string): string {
+  let stripped = text
+  let previous = ''
+  do {
+    previous = stripped
+    stripped = stripped.replace(/<[^>]+>/g, '')
+  } while (stripped !== previous)
+  return stripped
+}