chartlet

chartlet compiles a small JSON chart specification into a finished, accessible chart at build time. You get plain SVG, or an HTML figure with caption, source and data table. Nothing runs in the browser: no chart JavaScript, no hydration, no layout shift.

Website and documentation: casoon.github.io/chartlet

• Accessible by default: every chart carries a title and a generated description of the data, and the HTML output always includes the complete data as a table.
• Deterministic: the same specification produces the same bytes, so charts can be reviewed, diffed and cached like any other build artifact.
• Honest about problems: invalid input is rejected with a code, a path and a fix; layout compromises such as shortened labels are reported as warnings instead of happening silently.

Status: early alpha. Bar charts (single and grouped, vertical and horizontal), categorical line charts, time series with uncertainty bands and reference lines, warming stripes, calendar heatmaps, range bars and small multiples are supported. The specification may still change before the first stable release.

Quick start

Install the CLI from crates.io (Rust 1.88 or newer). While chartlet is in alpha, name the version explicitly:

cargo install chartlet --version 0.1.0-alpha.4

Save a minimal specification as spec.json:

{
  "schemaVersion": 1,
  "type": "bar",
  "title": "Monthly revenue",
  "data": [
    { "label": "January", "value": 120 },
    { "label": "February", "value": 180 },
    { "label": "March", "value": 150 }
  ]
}

Render it:

chartlet render spec.json --format html -o chart.html

To build from a clone of this repository instead, run cargo install --path ..

Website and documentation

The project website has the documentation, every example chart with its specification next to the rendered output, and the support matrix for embedding contexts, browsers, and screen readers.

The site is built with Astro on the shared CASOON Pages theme and renders every chart with @casoon/chartlet at build time. Its source lives in site/, the documentation in docs/.

Chart types

Each example has its rendered .svg and .html next to it. The SVG files are also the reference output of the test suite.

Missing values

In a line chart, null marks an observation that does not exist. chartlet leaves a visible gap instead of drawing a line across it. In a grouped bar chart, null leaves out that bar. The data table shows these cells as “Missing”. A single-series bar chart requires a value for every category.

Several series

Replace data with categories and series. Every series needs exactly one value per category. A legend is added automatically, and the data table gets one column per series.

Time series

"type": "time" replaces categories with timestamps and draws lines on a calendar axis. Each observation is { "time": …, "value": … }, where time is Unix seconds or an ISO 8601 date; a timestamp without its own offset is read as wall-clock time in timeAxis.timezone (a fixed UTC offset, UTC by default). Layers live in panes and may name their own color. See the time series guide for the contract, the theme, and the measured limits for dense series.

A line layer can carry an uncertainty band: give every point lower and upper. With "modeled": true the line is dashed, its band hatched, and legend, description and data table say “modeled”. An "annotation" layer draws a labelled reference line: value for a horizontal threshold, time for a vertical marker. A bare year such as "1850" is a valid timestamp, and annual data is labelled by year. "type": "multiples" lays out 2 to 12 titled panes as small time charts in a grid (columns), sharing one value axis and one legend.

Stripes, calendars and range bars

"type": "stripes" draws one stripe per year (stripes.firstYear, stripes.values); "type": "calendar" one cell per day of calendar.year (layout: months or weeks). Both color on a diverging scale of eight equal steps either side of reference (default 0), whose outermost steps begin at min and max (default: the largest distance from the reference). A missing value leaves an empty stripe or an outlined cell. "type": "rangebar" draws one span per entry of ranges (low, high, optional mid and modeled), vertical or horizontal.

{
  "schemaVersion": 1,
  "type": "bar",
  "title": "Budget and actual costs",
  "categories": ["January", "February", "March"],
  "series": [
    { "name": "Budget", "values": [120, 150, 140] },
    { "name": "Actual", "values": [130, 145, null] }
  ]
}

Interaction without JavaScript

The HTML output can add native controls and CSS-based interaction. No chart JavaScript is shipped.

• Series filter: a grouped chart gets a checkbox per series. Deselecting one hides its bars and value labels with CSS :has(); the axis does not rescale. The data table and description always show the full data. Browsers without :has() support simply keep every series visible.
• Zoom steps: add two to four zoomSteps to pre-compute narrower views of the same chart. The HTML output renders one variant per step and switches between them with radio buttons. Each step costs its own SVG in the output file.
• Tooltips: every bar and point carries a native <title> (Month: value, or Month – Series: value) that browsers can show on hover. The chart description and data table, rather than these hover-only tooltips, remain the assistive-technology alternative.

The pure SVG profile stays a single static chart; filtering and stepped zoom are HTML-only.

{
  "schemaVersion": 1,
  "type": "bar",
  "title": "Quarterly revenue",
  "data": [
    { "label": "Q1", "value": 320 },
    { "label": "Q2", "value": 345 },
    { "label": "Q3", "value": 380 },
    { "label": "Q4", "value": 410 }
  ],
  "zoomSteps": [
    { "label": "First half", "from": 0, "to": 1 },
    { "label": "All", "from": 0, "to": 3 }
  ]
}

Specification

schema/chartlet.schema.json is the complete contract and can be used for editor validation.

Limits: up to 100 categories and four series. Labels must be unique. Values must be zero or have a magnitude between 1e-100 and 1e100. A time chart carries up to four layers of 2000 observations each in one pane; markers and value labels are drawn up to 60 observations per layer, and beyond one observation per plot pixel the rendering is reported as dense_chart.

Output

Pass - instead of a file name to read the specification from standard input.

The SVG scales with its container, uses CSS classes for all styling and embeds no fonts, scripts or external resources.

Accessibility

• The SVG is exposed as a single image (role="img"). Its accessible name is the title, and its description is generated from the data, for example: “Bar chart with 4 categories and 2 series (Budget, Actual). Highest: 160 (Budget in April). Lowest: 120 (Budget in January). 1 value is missing.”
• The HTML output adds a <figure> with caption and a real <table> containing every value, including values whose visual label had to be left out.
• Series colors stay distinguishable for the common forms of color-vision deficiency and have at least 4.5:1 contrast against white — and against the dark theme's background, where the lowest series color reaches 7.65:1. The legend lists series in the same order as the bars.
• Text is never removed silently: shortened labels and omitted value labels produce warnings, and the full text stays in the specification and the data table.

Testing with VoiceOver and NVDA is still pending. Until then, treat the output as designed for accessibility, not as verified.

Warnings and errors

Errors stop rendering and name a code, the location in the specification as a JSON Pointer (/ for the whole document) and a way to fix it:

chartlet: series_length_mismatch at /series/0/values: expected 2 values, one per category; use null for a missing value

Warnings are written to standard error, and the chart is still produced:

Astro

The npm package @casoon/chartlet renders charts while Astro builds the site and ships no JavaScript to the browser. In the alpha, it calls the chartlet CLI, so install both:

npm install @casoon/chartlet@alpha
cargo install chartlet --version 0.1.0-alpha.4

---
import Chart from '@casoon/chartlet/astro';
import revenue from '../data/monthly-revenue.json';
---

<Chart id="monthly-revenue" spec={revenue} />

The CLI must be on PATH, or CHARTLET_BIN must point to it.

Rust

cargo add chartlet@0.1.0-alpha.4

use chartlet::{render_json, RenderFormat, RenderOptions};

let spec = std::fs::read_to_string("examples/monthly-revenue.json")?;
let output = render_json(&spec, RenderFormat::Html, &RenderOptions::default())?;
for warning in &output.warnings {
    eprintln!("{} at {}: {}", warning.code, warning.path, warning.message);
}
std::fs::write("chart.html", output.content)?;

render_with_metrics accepts your own TextMetrics implementation if your pages use a font whose widths differ noticeably from the built-in profile.

When chartlet is not the right tool

• You need continuous zooming, panning, cross-filtering, live updates or keyboard-addressable details for individual marks.
• The data changes at runtime rather than at build time.
• You need maps, networks, 3D charts or chart types beyond the ones listed above.
• You want to explore data rather than publish a finished chart.

License

MIT