git:20261030.73c8dd9 to git:20261031.7775fb5

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name: dataviz

description: Use this skill whenever you are about to create ANY chart, graph, plot, dashboard, or data visualization, in ANY output medium (HTML/React artifact, inline SVG, matplotlib/plotly/d3/Recharts, PNG). Read it BEFORE writing the first line of chart code, choosing chart colors, building a stat tile / KPI row, or laying out a dashboard. Teaches a design-system-agnostic method - form heuristic, color formula with a runnable validator, mark specs, interaction rules. Default palette in references/palette.md.

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# 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 Delta E.** `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 Delta E >= 8 is the target (OKLab x100); 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) |