Color Accessibility

Assign color by role:

• Neutral colors for most data — the default state of a chart is quiet.
• One emphasis color for the primary focus.
• Semantic colors only when meaning is stable and culturally appropriate.
• Sequential scales for low-to-high values; diverging scales for values around a meaningful center; categorical palettes for unordered groups.

Rules:

• Never use color as the only carrier of status or category (WCAG 1.4.1, Level A). Reinforce with labels, position, shape, pattern, or icons.
• No rainbow scales — they aren’t perceptually ordered and create false boundaries.
• Categorical palettes: target ≤6 distinct hues, 8 as a hard ceiling. Beyond that, group into “Other,” facet into small multiples, or change encodings. CVD-safe categorical palettes realistically max out at 4–6 distinguishable hues (this is why ColorBrewer’s colorblind-safe filter eliminates most large qualitative schemes).
• Keep the same category color consistent across related views.
• Don’t assign red/green automatically when better-or-worse is ambiguous — and remember red-green confusion is the most common CVD.
• Preserve legibility in grayscale where practical.

Color-vision deficiency

Red-green CVD affects roughly 1 in 12 men (~8%) and 1 in 200 women (~0.5%) of Northern European descent; prevalence is lower in other populations — don’t state the 8% figure unqualified. Deuteranomaly is the most common form.

Test palettes with a simulator: Viz Palette (projects.susielu.com/viz-palette) simulates deuteranopia/protanopia/tritanopia and flags confusable pairs at realistic mark sizes; ColorBrewer (colorbrewer2.org) has a colorblind-safe filter. A “safe” palette does not excuse redundant encoding — WCAG 1.4.1 still applies.

Contrast requirements (WCAG 2.2, Level AA)

Adjacent stacked segments or pie slices that don’t reach 3:1 against each other need separators or borders. Avoid extremely thin lines for important information, rotated text where possible, and any text below 12px (Chartability’s floor — stricter than WCAG, which has no font-size minimum, but the right bar for charts).

Interaction accessibility

• All chart functionality — tooltips, filtering, zoom, selection — operable by keyboard alone (WCAG 2.1.1), with a visible focus indicator (2.4.7).
• Tooltips must be dismissible, hoverable, and persistent (1.4.13) — a tooltip that vanishes when the pointer moves toward it fails AA.
• Never put essential information only in a hover state.

Nonvisual access

Provide:

• A concise text summary of the main finding.
• Alt text for each meaningful visual (see below).
• A data table or accessible equivalent when exact values matter — a real HTML table adjacent to the chart (optionally collapsed) is the most robust screen-reader path. For SVG charts, the production-proven pattern is role="img" + a takeaway-bearing accessible name + a separate HTML table; in-SVG ARIA table semantics have inconsistent screen-reader support.
• Logical keyboard and reading order; descriptive titles for visuals and controls; text equivalents for interactive states.

Writing alt text

Two complementary models, both worth knowing:

• Cesal’s formula (Amy Cesal, Nightingale 2020): [Chart type] of [type of data] where [reason for including the chart / takeaway], with the data source linked in surrounding text. Keep it short — screen readers read alt text linearly with no ability to scan back.
• The four-level model (Lundgard & Satyanarayan, IEEE VIS 2021): L1 chart type/axes/encodings → L2 statistics (extrema, outliers, comparisons) → L3 trends and patterns → L4 domain context. Their study found blind and sighted readers both rank L3 (the trend) most useful, and blind readers value L4 interpretation least. So: lead with the trend and key statistics; never stop at L1 (“bar chart of sales by region” alone fails the reader); keep editorializing out.

House pattern combining both:

[Chart type] showing [measures] across [dimensions and period]. Main pattern: [takeaway]. Notable values or exceptions: [details]. Source and limitations: [source/caveat].

Quick audit (adapted from Chartability’s critical tests)

Chartability (Frank Elavsky, chartability.fizz.studio, CC-BY-SA) extends POUR with three viz-specific principles — Compromising (tensions), Assistive (reduces load), Flexible (adapts to preferences) — across 50 heuristics, 14 critical. The critical ones to check on every deliverable:

• Geometries and large text ≥3:1 contrast; regular text ≥4.5:1.
• Information available without visuals (screen-reader path exists).
• A human-readable data table is provided.
• Anything mouse-interactive is also keyboard-interactive.
• A title, summary, or caption exists.
• No text below 9pt/12px.
• Color never carries meaning alone.