feat: add Image-Charts backend (hosted-image-URL render target)

[Francois-Guillaume Ribreau (FGRibreau)]

What

Adds an image-charts backend to flint-js that compiles the shared core semantic layer into a single Image-Charts chart URL. That URL is a hosted image that renders server-side and embeds anywhere an <img> works (email, PDF, Slack, Notion, no-code) with no runtime JavaScript. It follows docs/adding-a-backend.md and mirrors the Excel backend, with an inline assembler that returns a declarative artifact ({ type: 'image-charts', url }) instead of a client-side JS spec, and gates chart types to the ones with a faithful mapping.

Why

The existing backends emit a JS spec that a local renderer draws. That leaves out the case where a chart needs to live as a durable, shareable URL (an email, a PDF, a Slack or Notion embed, a no-code image control), where there is no JS runtime and the image has to persist. This backend covers that case by targeting a hosted renderer.

Design / scope

• Pure and offline. assembleImageCharts builds a string, with no network I/O, no crypto, and no new dependencies. The data reaches Image-Charts only if something later loads the <img>, which is the caller's choice, the same way choosing the Excel backend chooses Office.js. The library keeps its no-I/O property.
• Free tier only. It emits unsigned URLs (no account/HMAC params). Signed enterprise URLs need a server-side secret, which has no place in a pure compiler function.
• Reuses the real pipeline. It runs the same Phase 0 semantic resolution and banded-axis overflow filtering as the other backends, then serializes into the Image-Charts query grammar (cht, chd=a:, chs, chxt/chxl, chco, chdl, chm, chtt).
• Partial coverage by design, like Excel. Bar, grouped bar, stacked bar, line, sparkline, area, scatter, pie, donut, and radar map to a faithful cht. Unsupported types (Boxplot, Sankey, Heatmap, and so on) throw with a clear message.

Tests

• npm run typecheck, npm run lint, npm run build are clean.
• npm run test: 947 passed, including 2 new public-API smoke assertions (URL shape, and a throw on unsupported types).
• A new test-data generator (Image-Charts: Core Templates) exercises the distinct URL param paths.

Disclosure

I maintain Image-Charts, a commercial hosted chart-rendering service. Its free tier (unsigned URLs, watermarked) is what this backend emits. If you would rather not carry a third-party render target in-tree, I am happy to maintain this as an external companion package instead.

[Chenglong Wang (Chenglong-MS)]

This is great! Let's add this to the repo. I think we can first merge it to dev, then I'll propose a backend extension folder to host your backend.