Free chart image API — line, bar and sparkline charts from numbers in a URL

You have five numbers and you want a chart — in a README, a markdown doc, an email, a chat message, a status page, an admin screen you don't want to add a chart library to. Wiring up Chart.js (bundle, canvas, config object, build step) for one <img>-sized picture is absurd. The URL-parameter pattern fixes it: put the numbers in a query string, get image bytes back.

Mockbird serves charts exactly that way — no key, no signup, no watermark, no JSON config to hand-write, and the same URL returns the same bytes forever, so caches and CDNs love it:

The 10-second version

# a 600x300 line chart PNG — this URL works right now
curl -o chart.png "https://mockbird.mockbird.workers.dev/m/demo/chart?data=3,7,4,9,6"

# bar chart with labels and a title
curl -o chart.png "https://mockbird.mockbird.workers.dev/m/demo/chart?data=12,19,7,24&type=bar&labels=q1,q2,q3,q4&title=Signups"

# compact README sparkline (no axes, dot on the last point)
curl -o spark.png "https://mockbird.mockbird.workers.dev/m/demo/chart/300x80?data=3,7,4,9,6,2,8&type=spark"

# vector output (crisp at any size)
curl -o chart.svg "https://mockbird.mockbird.workers.dev/m/demo/chart.svg?data=3,7,4,9,6&type=area"

Or straight in markdown — the README case:

![requests this week](https://mockbird.mockbird.workers.dev/m/demo/chart/300x80?data=3,7,4,9,6,2,8&type=spark)

All the parameters

Charts people actually make this way

# uptime / response-time sparkline in a status README
…/chart/300x60?data=212,198,205,301,220,215,208&type=spark&color=10b981

# a dark-mode dashboard tile
…/chart?data=40,42,39,45,50,48,55&theme=dark&title=Weekly+active+users

# fixed y-range so consecutive screenshots are comparable
…/chart?data=71,74,69,80&min=0&max=100&type=bar&labels=mon,tue,wed,thu

# transparent background to sit on any page color
…/chart/400x150?data=3,7,4,9,6&type=area&bg=transparent

Numbers straight from a shell pipeline work too: data=$(printf '%s,' "${values[@]}") or a jq -r 'map(.count)|join(",")' away from any JSON. Because output is deterministic, re-running produces byte-identical files — safe for build pipelines and cached embeds.

Honest comparison

Being upfront: QuickChart is excellent and open-source — if you need pie charts, multiple series, legends, or Chart.js-grade typography, use it (or self-host the AGPL server) and pay for volume if you outgrow the free 1,000/month. Image-Charts keeps the old Google Image Charts URL syntax alive, which matters if you're migrating code written before Google turned that API off in 2019 (chart.googleapis.com now 404s). Mockbird's angle is the zero-config tier: numbers in, picture out, no JSON to hand-write, no watermark, no monthly cap — and it lives next to your mock REST API, placeholder images, avatars, QR codes and OG cards under one project URL. For interactive charts in a real app, use a real chart library.

Pairs well with the rest of your mock backend

Seeded mock data + chart endpoint composes with plain string interpolation — an admin-dashboard prototype with zero chart code:

// chart the demo API's own product prices
const res = await fetch("https://mockbird.mockbird.workers.dev/m/demo/products?limit=12&select=price");
const prices = (await res.json()).map(p => p.price);
document.body.innerHTML =
  `<img src="https://mockbird.mockbird.workers.dev/m/demo/chart/500x200?data=${prices.join(",")}&type=bar&title=Prices">`;

Limits

• 1–300 data points, data string ≤4000 chars. One series per chart today — pipe-separated multi-series is on the list; tell us if you need it.
• PNG capped at 1,000,000 pixels; use ?format=svg for larger.
• PNG text uses a deliberately blocky pixel font (same as /img) — legible and honest about being a placeholder; the SVG twin uses a real system font.
• Counts toward the project's daily request cap (the shared demo project has a generous one; your own project gets its own).

Full reference in the docs. More image guides: placeholder image API · PNG placeholder API · OG image API · QR code API · avatar API. Create your API →