Bolding three to five lines at once leaves readers with a heap of emphasis and no real emphasis

What it is

Highlighting is a scarce resource: the more elements a chart promotes, the less each promotion is worth. Once three to five lines are all bolded and saturated, readers face "a heap of emphasised things" and nothing actually stands out — contrast is relative, and when the "emphasised layer" outnumbers the background, the chart has simply exchanged one background for another.

Why it happens

The failure is the same relative computation as ever: prominence depends on the ratio of target to context, and once highlighted items become the majority the ratio flips — attention can land on the few un-highlighted elements instead, the exact opposite of the design intent. Attention allocation degrades too: visual attention has finite capacity, and spread across five or six targets it is divided almost evenly, so none is examined deeply. Perceptually, "highlighting five" approximates "highlighting one plus four plain elements," and in most cases honestly highlighting just the one beats that arrangement.

Where it stops holding

Multi-target highlighting has two legitimate forms: first, when the highlighted set genuinely is a group (say, every qualifying series, with few failures) — the group contrasts as a whole against the background and holds; second, faceted or small-multiple layouts highlighting a different target per panel — panels do not compete, and the same target can even be compared across panels. The real red line is a single chart where highlighted items approach or exceed the background count — that is not emphasis, it is a repaint. Interaction offers the elegant exit: hover highlights the current item, and multi-target demand is served by repeated hovers or brush selection.

Applying it

• Cap single-chart highlights at one or two items; needing more means the chart's task needs restructuring (facets, sorting, or interactive filtering).
• When a group genuinely warrants emphasis, treat the group as one unit: uniform inside, uniform outside, clear between.
• Verification: ask readers "what is this chart emphasising?"; more than two answers or contradictions mean highlight overload — cut to one or two, or move to interactive highlighting.

Related

• Same group: U5.05.1 Highlighting works through contrast with the rest, not through its own intensity · U5.05.2 Dimming the rest disturbs the whole less than amplifying the target · U5.05.3 De-emphasised elements keep a readable minimum contrast · U5.05.5 State the basis for a highlight, or it reads as a data property
• Nearby: U5.04.4 Side-by-side charts need a shared order for cross-chart comparison · U5.02.5 Interactive legends double as filters, but clickability must be signalled
• Search terms: highlight overload · attention capacity · focus strategy