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chartjs2img

Error feedback (CLI) ​

A Chart.js config with a typo (e.g. "type": "pi" instead of "pie") renders a blank or partial image. Without console access you'd be left guessing why your chart is empty. The CLI captures the same messages you'd see in the browser's DevTools and streams them to stderr as the render runs.

The shape ​

Each message is prefixed with its level and the literal message text from Chart.js:

[chart ERROR] "pi" is not a registered controller.
[chart WARN]  Some warning message from Chart.js.

Two levels are emitted:

  • ERROR — Chart.js threw or refused to render the config.
  • WARN — Chart.js rendered something but flagged a likely mistake.

Example ​

bash
$ echo '{"type":"pi","data":{"labels":["A"],"datasets":[{"data":[1]}]}}' \
  | chartjs2img render -o chart.png

[chart ERROR] "pi" is not a registered controller.
Written to chart.png (hash: 1234abcd5678efgh)

Notice: the render still completes (writing an empty or partial PNG) and the CLI still exits 0. Chart.js warnings are not treated as CLI failures, so pipelines don't fail on a typo — you have to watch stderr and decide yourself.

Piping the image while capturing messages ​

Since stdout carries the image binary and stderr carries diagnostics, you can redirect them independently:

bash
chartjs2img render -i chart.json 2> errors.log > chart.png

Check errors.log for messages and chart.png for the image.

When rendering "succeeds but is wrong" ​

A Chart.js config can be syntactically valid, render, and still be wrong — e.g. a scatter chart given labels + datasets[0].data: [1,2,3] instead of {x,y} pairs. These often don't throw but emit a warning. Always scan stderr before declaring success in a CI job.

If you're driving chartjs2img render from an LLM agent:

  1. Run the render, capturing stderr.
  2. If stderr is non-empty, feed the messages back into the agent and ask it to fix the config.
  3. Loop until stderr is clean.

This is effectively how the chartjs2img-render Agent Skill works. See AI Guide → Claude Code plugin for the full loop.

Common messages ​

MessageUsual cause
"X" is not a registered controller.Typo in chart.type, or a plugin chart type missing from the bundle.
Cannot read properties of undefined...Missing datasets, wrong dataset shape, or numeric data where object expected.
No dataset matched the indexMismatch between labels.length and data.length.
The scale "y" doesn't have a parserTime-series data without a time adapter — use scales.x.type: "time".

When in doubt, the full Chart.js error reference tells you what each message means.

Non-Chart.js errors ​

Messages about Chromium (download, launch, container permissions) appear without the [chart …] prefix and typically exit non-zero. They signal an environment problem, not a config problem:

[renderer] Chrome/Chromium not found. Installing Chrome for Testing...
[renderer] Downloading Chrome 123.0.0.0 for mac-arm64...

See Install → Chromium / Chrome detection if the auto-install doesn't work for your platform.

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