Browser extension and userscript
The plotdown extension for Chrome, Edge and Firefox draws plot, oplot and observable-plot blocks on the pages you read, where the site does nothing for them: GitHub issues, pull requests, comments, READMEs and gists; GitLab, gitlab.com or self-hosted; Bitbucket; Gitea, Forgejo and Codeberg; Azure DevOps; the answers of ChatGPT, Claude and Gemini; any page that shows Markdown code blocks. Only you see the charts: the page does not change for other readers.
Warning
The extension is not in the Chrome Web Store, Edge Add-ons or Firefox Add-ons yet. Today, build it from the repository and load it unpacked, or use the userscript.
What it does
It is plotdown.js, the script a site owner adds with one line, put in the page by your browser instead. A small detector runs on every page; only a page with a candidate block receives plotdown.js, which draws each block before it, with a Source button. Remote data stays off whatever the page says. The toolbar button turns plotdown off for the current site, or everywhere. The extension collects nothing and sends nothing: see its privacy policy.
Load it unpacked
From a clone of the repository, with Node 24 and pnpm 10:
pnpm install
pnpm build # writes packages/extension/dist/- Chrome or Edge: open
chrome://extensions(oredge://extensions), turn on Developer mode, press Load unpacked and pickpackages/extension/dist/chrome. - Firefox (153 or later): open
about:debugging#/runtime/this-firefox, press Load Temporary Add-on and pickpackages/extension/dist/firefox/manifest.json. A temporary add-on is removed when Firefox closes.
The browser warns that the extension can read all sites: that is what lets it work on a self-hosted GitLab or any other page without setup. A variant limited to the known sites is built with node build.mjs --sites=known in packages/extension; it asks for any other site when you turn it on there.
The userscript
pnpm build also writes packages/extension/dist/plotdown.user.js: plotdown.js under a userscript header (@match *://*/*, @grant none), with the same detector, for Tampermonkey, Violentmonkey or Greasemonkey. Open the file in the browser with the manager installed, or paste it into a new script. Use the manager's per-site exclusions to turn it off somewhere. It does not update by itself: build it again for a new version.
Check it
Open a page with a block, such as the readme of plotdown on GitHub: the blocks above the committed images become live charts with tips, and a Source button. A real gnuplot block on GitHub, whose markup is that of a plot block, stays code.
More, including the tests and the store packages: packages/extension.
Note
Written from the extension's documentation; this page was not tried by hand. The extension's own browser tests load it unpacked in Chromium and in Firefox: github.com, a page under a strict CSP, a streamed block, the switches, and the userscript.