A tooltip showing only a bare number forces readers to guess what series or category it belongs to

What it is

A tooltip's minimum qualifying content is "what this is + its value": a tooltip showing only the number — a lone "42" — forces readers to reverse-engineer from the graphic position what they are looking at, and that inference fails reliably in multi-series, multi-category charts. Identity (series, category, time point) and value (with units) are first-class tooltip citizens; everything else is second-class.

Why it happens

Identity anchors the tooltip back to data semantics: the reader's question on hover is "what is this point's value," and the question carries a subject — which series, which category, at which time. The graphic answers part of the subject (position, colour), but colour requires legend lookup and position requires axis reading; only a tooltip supplying the full subject yields a self-sufficient sentence ("East region, March sales: 1,204 units"). Identity-less tooltips also break shareability — the user screenshots the tooltip for a colleague who has no graphic context, and "42" means nothing. The tooltip structure should therefore be templated: identity line first, then value with units, remaining fields folded, with one uniform template so readers build a reading habit.

Where it stops holding

Identity has a redundancy boundary: when the tooltip only ever appears on a single-series chart whose title names the series, repeating the series name is noise — the requirement is self-sufficiency, not completeness. In dense multi-series charts, identity is doubly critical and pairs well with "highlight the corresponding series," letting graphic and tooltip corroborate each other. Time granularity should be explicit ("March," not a bare number), units must match the chart's axis, and formatting must follow product-wide conventions — the tooltip is UI grown from the chart, not an independent component.

Applying it

• Fix the tooltip template: identity line (series / category / time) + value line (number with unit) + optional folded detail.
• Highlight the corresponding mark when the tooltip appears, building visual cross-reference.
• Verification: crop a screenshot containing only the tooltip and hand it to a colleague — can they independently state "what, how much, when"? Failure means identity is missing; fix the template.

Related

• Same group: U6.09.1 Tooltips carry exact values · U6.09.2 Touchscreens have no hover and need alternative paths
• Nearby: U4.06.5 Semantic colour needs a text label to be judged on its own · U5.01.2 Direct labelling eliminates the round-trip
• Search terms: tooltip structure · identity plus value · hover template