Chart series need pattern or direct labels
Aliases: direct labeling · hatching · chart without color
What it is
Several lines or grouped bars that enter the legend by hue alone collapse when photocopied, when color vision differs, or when the stroke is thin. Series need pattern, line style, or names written on the marks: dash versus solid, hatching, end-of-line labels. A legend may stay. It cannot be the only index (“remember: blue is North America”). How status lights get icons, and how links get underlines, are not this leaf.
Why it happens
“Color plus legend” asks for two jobs at once: map hue to a name in the legend, then hold that mapping on the plot. Working memory is small; more series drop the mapping first. If the hue channel also breaks, there is nothing to check against. Direct labels bind the name to the mark and cancel the extra lookup. Texture and dash are spatial features; they survive gray print and color-vision differences.
Adjacent hues on thin strokes or small patches (blue/purple, green/cyan) are hard even for typical color vision — a different layer from “has a deficiency or not.” Hover-only series names lock the information behind a pointer; keyboard and touch never meet “that stroke.” Complex graphics still need a long description for the trend; that is a later step. This leaf only requires that series identity be visible on the chart, not a recipe for writing the data table.
Studying it
Print gray or strip color, cover the legend, and ask which stroke is the treatment group. Compare hue only, hue plus dash/texture, and direct end labels.
Independent variables: encoding, series count, stroke or bar width. Dependent variables: series identification accuracy, whether the legend must be consulted, accuracy in gray.
Do not let one simulator filter replace a gray test: the simulator can pass and the copier still fail.
Where it stops holding
A single-series chart has no identity problem; axes and values still need to be readable. Quantitative color scales (heatmaps, terrain) are not categorical palettes; hatching does not help — use numeric ticks on the legend, contours, or queryable values. Too much texture on a map becomes noise; prefer direct labels plus togglable layers. If a series enters by changing color in motion, in-between frames still need the pattern.
Applying it
- Differentiate lines with dash and endpoint marks, and write the series name at the end of the line so the legend swatch is not the only key.
- Hatch bars and areas. Do not only swap fill hue.
- Tooltips are extra. Series identity must hold on a still frame.
- Verify: gray the screenshot, cover the legend, ask “which series is this.” Misses get texture or a name on the mark. Print a gray page and confirm a copier can still split them.