Check a palette against WCAG contrast thresholds — a11y_check

Computes the contrast ratio of every color against a reference background and reports whether each meets the WCAG threshold (4.5:1 for AA, 7:1 for AAA). When alpha < 1, contrast is computed against the alpha-composited rendered color – the color the viewer actually sees. Works on any palette – viridis, RColorBrewer, plotly's discrete sequences, or a custom hex vector.

Usage

a11y_check_palette(colors, bg = "#ffffff", level = "AA", alpha = 1)

Arguments

colors: Character vector of hex codes.
bg: Reference background hex(es). Pass a single value (e.g. "#ffffff") or a vector (e.g. c("#ffffff", "#1a1a1a")) to verify against multiple backgrounds. Returns one row per (color, bg) pair.
level: "AA" (4.5:1) or "AAA" (7:1). Use "AA-large" (3:1) for non-text elements such as data marks, axis lines, and focus rings.
alpha: Opacity in [0, 1]. 1 (default) checks the raw color. Values below 1 composite each color over bg first, then check the rendered result – useful when chart geoms use alpha < 1.

Value

Data frame with columns color, bg, alpha, rendered, ratio, status.

Examples

a11y_check_palette(c("#000000", "#cccccc", "#ff0000"))
#>     color      bg alpha rendered ratio status
#> 1 #000000 #ffffff     1  #000000 21.00     ok
#> 2 #cccccc #ffffff     1  #cccccc  1.61   todo
#> 3 #ff0000 #ffffff     1  #ff0000  4.00   todo
a11y_check_palette(c("#0072B2"), bg = c("#ffffff", "#1a1a1a"), level = "AA-large")
#>     color      bg alpha rendered ratio status
#> 1 #0072B2 #ffffff     1  #0072B2  5.19     ok
#> 2 #0072B2 #1a1a1a     1  #0072B2  3.36     ok
a11y_check_palette(c("#0072B2", "#D55E00"), alpha = 0.7, level = "AA-large")
#>     color      bg alpha rendered ratio status
#> 1 #0072B2 #ffffff   0.7  #4C9CC9  3.04     ok
#> 2 #D55E00 #ffffff   0.7  #E18E4C  2.57   todo