| name | Data Visualization |
| description | Produce a chart, graph, dashboard, or any data visualization that reads as one system - elegant, accessible, and consistent in light and dark - BRAND-NEUTRAL, shipping a placeholder palette to swap for your own. Read this BEFORE generating ANY chart (bar, line, area, heatmap, scatter, sparkline, donut), choosing chart colors, building a stat tile / meter / KPI row, or laying out a dashboard. Teaches a design-system-AGNOSTIC method: a form heuristic, a color formula with a runnable validator, mark specs, and interaction rules. The method is invariant; a design system plugs in its own ramps and surfaces. A validated default palette is documented in `references/palette.md` -- swap that file's values for your brand's. Triggers on: "chart", "graph", "plot", "data viz", "dashboard", "analytics", "visualize data", "categorical colors", "sequential / diverging palette", "stat tile", "sparkline", "heatmap", "legend", "axis", "tooltip", "chart colors", "color by series".
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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.
- 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
- 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
- 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.
- 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
- 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
- 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.
- 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) |
