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 ×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)