Accessibility

Charts published on the web are subject to WCAG 2.2 Level AA. These rules make that concrete for data graphics. IDs: AX-*.

1 Contrast

AX-1 (MUST) Text on and around charts meets 4.5:1 contrast against its background (WCAG 1.4.3); large text (≥24px, or ≥18.66px bold) may drop to 3:1. The neutral text colors in 03-color-palettes.qmd comply on white (#333333 = 12.6:1, #555555 = 7.5:1).
AX-2 (MUST) Essential graphical elements — bars, lines, points that the reader must distinguish to understand the chart — meet 3:1 against adjacent colors (WCAG 1.4.11). The default categorical palette was chosen to satisfy this against white (verified 3.03–10.86:1).
AX-3 (SHOULD) Sequential ramps cannot keep 3:1 between adjacent steps; when a chart depends on fine sequential distinctions, provide the values as labels or an accompanying table (UK Analysis Function requirement). Note also Datawrapper’s caution that mechanically applying WCAG ratios to gradients can reduce discriminability — satisfy the intent (readable data) and document exceptions.

AX-4 (MUST) Color is never the only carrier of meaning (WCAG 1.4.1). Pair it with direct labels (TY-6), position, marker shape, or line type. Okabe & Ito: “use not only different colors but also a combination of different shapes, positions, line types.”
AX-5 (MUST) Test every published palette combination with a color-vision simulator (e.g. Sim Daltonism, Viz Palette) and in greyscale; adjacent categories must differ in lightness, not hue alone. Avoid red/green pairs; prefer blue/orange or blue/red; use vermilion rather than pure red where red is required (Okabe–Ito).

AX-6 (MUST) Every chart has a text alternative serving the equivalent purpose (WCAG 1.1.1): alt text on the image, plus either an accessible data table or descriptive prose for complex charts (UK Analysis Function).
AX-7 (MUST) Alt text states the chart type, the variables, and the finding — content and function, not appearance. Keep it roughly a sentence (~160 characters, Urban Institute); put the full data elsewhere. No “image of”, no “chart of” prefix beyond the type. Example: “Line chart showing UK inflation falling from 11% in 2022 to 2% by mid-2024.”
AX-8 (SHOULD) Render titles, subtitles, and sources as HTML text, not pixels (TY-5), and publish charts as SVG where feasible (UK AF) so text scales and high-contrast modes work.
AX-9 (SHOULD) Offer the underlying data for download with unique, descriptive link text (UK AF).

AX-10 (MUST) Minimum rendered text size on the web: ~11–12px for source notes, 12px for axis labels (Urban Institute scale, TY-3); verify at the smallest responsive breakpoint, not the desktop layout.
AX-11 (SHOULD) Lines carrying data are thick enough to show their color (roughly ≥2px); increase mark weight rather than saturating color to rescue a thin mark (Okabe–Ito: bold fonts, thicker lines for color-coded objects).