Dataviz

---
name: dataviz
description: Use when creating data visualizations, charts, graphs, stat tiles, or analytics dashboards
---

# Data Visualization

A chart is **read by people and executed by you**. This skill turns "make it look
good" into a procedure with checks, so the result is right by construction rather
than by taste.

**The method here is design-system-agnostic.** Nothing in the procedure, the form
heuristic, the six checks, or the mark specs is specific to one product. A design
system supplies a small set of *parameters* (its ramps, a categorical order, a
diverging pair, a status palette, a texture, its surfaces, its filter components);
the method consumes them unchanged. A **validated default palette** is the
reference instance, fully specified in `references/palette.md`. To target your
brand, read that file's structure and substitute its values — touch nothing else.

> The single most important habit: **the color part is computable, so compute it.**
> Never eyeball whether a palette is colorblind-safe — run `scripts/validate_palette.js`.

## The procedure — do these in order

Color comes LAST. Most bad charts pick colors first.

1. **Pick the form.** What is the data's job — magnitude, identity, polarity, a
   single headline, change-over-time? The job picks the chart type, and sometimes
   the answer is *not a chart* (a stat tile or hero number). → `references/choosing-a-form.md`
2. **Assign color by the job it does.** Categorical (identity), sequential
   (magnitude), diverging (polarity), or status (state) — each has one rule.
   Assign categorical hues in fixed order, never cycled. → `references/color-formula.md`
3. **VALIDATE the palette — run the script, don't reason about ΔE.**
   `node scripts/validate_palette.js "<hex,hex,…>" --mode light` (relative to
   this skill's base directory — or load it as `<script type="module">` in the
   chart's own page, where it reads
   `data-palette` off `<body>` and logs a `console.table` report). It returns
   pass/fail on the lightness band, chroma floor, adjacent-pair CVD separation,
   the normal-vision floor, and contrast. Fix anything that FAILs before continuing. Re-run for
   `--mode dark` with that mode's surface.
4. **Apply mark specs & spacers.** Thin marks, 4px rounded data-ends anchored to
   the baseline, 2px lines, ≥8px markers, a 2px surface gap between fills (stacked
   segments and adjacent bars alike) and a 2px surface ring on overlapping marks,
   selective direct labels. → `references/marks-and-anatomy.md`
5. **Add the hover layer — by default.** An HTML/SVG chart *is* interactive; ship
   a crosshair+tooltip on line/area and a per-mark hover tooltip on bar/dot/cell.
   The only form that skips it is a bare stat tile with no plot. Hit targets bigger
   than the mark; filters in one row above the charts. → `references/interaction.md`
6. **Final accessibility pass.** For ≥ 2 series a legend is always present and ≤ 4
   are also direct-labeled (a single series needs no legend box — the title names
   it), so identity is never color-alone; a table view exists; dark mode is **selected** — its own
   steps from the same ramps, validated against the dark surface, not an automatic
   flip; texture is available for the CVD/print/forced-colors case.
7. **Render it and look at it.** The validator checks color, not layout — open or
   screenshot the output and eyeball it for label collisions, geometry, and overflow
   before calling it done.

Then check the result against **`references/anti-patterns.md`** — it is the catalog
of what goes wrong. If your chart matches an entry, it's wrong.

## Non-negotiables (true in every design system)

- **Assign categorical hues in fixed order, never cycled.** A 9th series is never a
  generated hue — it folds into "Other," small multiples, or composite encoding.
- **One axis.** Never a dual-axis chart (two y-scales). Two measures of different
  scale → two charts, small multiples, or indexed to a common base. *(This is the
  #1 chart mistake — see anti-patterns.)*
- **Color follows the entity, never its rank.** A filter that changes the series
  count must not repaint the survivors.
- **Sequential = one hue, light→dark. Diverging = two hues + a neutral gray
  midpoint.** Never a rainbow; never a hue at the diverging midpoint.
- **Run the validator before shipping any categorical palette.** CVD ΔE ≥ 8 is the
  target (OKLab ×100); 6–8 is a floor that is legal ONLY with secondary encoding. A
  normal-vision floor below 15 is a hard FAIL — full-color readers can't tell the
  pair apart; re-step it on the adjacent pairlist (secondary encoding does not excuse
  this one); under `--pairs all` cut series or facet instead — see check 4. A contrast WARN
  obligates visible labels or a table view — it is not dismissable.
- **Thin marks; a legend always present for ≥ 2 series (none for one), with
  selective direct labels (never a number on every point); recessive grid/axes.**
- **Text wears text tokens, never the series color** — values, labels, and legends
  stay in primary/secondary/muted ink; a colored mark beside them carries identity.
- **Status colors are reserved** (good/warning/serious/critical) and never reused
  for "series 4"; they ship with an icon + label, never color alone.

## Plugging in a design system

The method is invariant; only these parameters change per system. The reference
instance — every value filled in — is `references/palette.md`.

| Parameter | What the system provides |
|---|---|
| **Ramps** | the hue scales (named steps) the palette draws from |
| **Categorical theme** | the fixed hue order (a named theme); default + alternates |
| **Sequential hue** | the default single hue for magnitude |
| **Diverging pair** | two warm/cool poles + a neutral midpoint |
| **Status palette** | good / warning / serious / critical — steps distinct from categorical |
| **Texture fill** | one directional hand-drawn fill, used at 45° / 135° |
| **Surfaces** | light & dark chart-surface colors (the validator needs these) |
| **Filter controls** | date-range & dimension controls (behavioral spec in `interaction.md`) |

To onboard a new system: fill those rows, feed its ramps to the validator, and let
it snap each slot to the nearest passing step. Structure and rules stay as written.

## Reference files

| File | What it answers |
|------|-----------------|
| `references/choosing-a-form.md` | Which chart type / is it even a chart? |
| `references/color-formula.md` | The four jobs, the six checks, snap-to-passing |
| `references/marks-and-anatomy.md` | Mark specs, spacers, labels, figures, hero number |
| `references/interaction.md` | Tooltips & hover, filters & time ranges |
| `references/components.md` | The pieces a chart is made of — build each in plain HTML |
| `references/anti-patterns.md` | **What goes wrong — check every chart against this** |
| `references/palette.md` | **The reference palette instance** — every parameter, filled in; swap for your brand's |
| `scripts/validate_palette.js` | Runnable six-checks validator (run it; don't eyeball) |