Past a dozen entries, matching a color to its legend item turns into a slow linear search

What it is

With few legend entries, readers lock onto a candidate at a glance; past a dozen or so, colours begin resembling one another, positional correspondence stretches and breaks, and matching degrades into a serial linear search — compare entry one, then entry two, until a hit. Cost grows linearly with entry count, error rates climb with it, and readers increasingly abandon the search midway and guess from impression.

Why it happens

Linearisation happens when two capacity limits break at once: working memory buffers only three or four candidate colours, so beyond that readers carry one or two features per trip and must re-check the rest repeatedly; meanwhile the number of similar-coloured pairs grows faster than the entry count, diluting the discriminable combinations and turning comparison from "find the different one" into "tell the near-identical ones apart." Together these dissolve the illusion of parallel search — the process becomes genuinely serial. That is the empirical ground for entry-count conventions.

Where it stops holding

Linearisation is gradual, not sudden: a value-sorted legend on an ordered graphic lets positional correspondence absorb part of the search cost; interactive search boxes and hover highlights bypass some comparison entirely. But every aid requires the reader to opt in — undiscovered, the linear search continues unchanged. The sensible ceiling depends on colour separation and reader familiarity; a dozen entries is a common warning line, not a hard threshold — fewer when colours are close, more when shape redundancy is strong.

Applying it

• Past ten entries, subtract first: merge the long tail into "Other," or split into small multiples.
• When many categories must stay, sort the legend by value and layer on shape redundancy so position and shape share the colour-comparison load.
• Verification: time readers finding a designated category; if time grows markedly with the entry's position in the legend (later items clearly slower), the search is linear — cut entries or add interaction.

Related

• Same group: U5.02.1 Legend-to-chart distance sets the cost of each round-trip · U5.02.2 Legend arrangement direction should match the graphic's spatial order · U5.02.3 Above, right, and below placements carry different matching costs · U5.02.5 Interactive legends double as filters, but clickability must be signalled
• Nearby: U4.01.2 Too many categories means grouping or aggregation · U5.06.2 Past the density ceiling, aggregate, sample, or facet
• Search terms: legend overload · serial visual search · category aggregation