One question, several viable charts — they differ in reading cost

What it is

"Task determines chart" does not mean one question, one unique answer. Under a composition question, pie, stacked bar, and treemap are all viable; under distribution comparison, box plots, violins, and beeswarms each have their strengths; for multidimensional relations, scatterplots, bubbles, and parallel coordinates are all legal. The difference within the viable set is reading cost: readout precision, ease of comparison, room to grow, audience graphicacy, layout price. Choosing a chart is therefore not a search for the single correct answer but a cost comparison within the viable set — more like picking a tool than grading true/false.

Why it happens

The viable set exists because one question can be served by several encoding combinations: composition needs the perception of "part against whole," and angle, share-of-length, and share-of-area all carry it — ranked differently in precision (length > area > angle). Reading cost has four components: encoding precision (readout variance of that channel), learning cost (how familiar the audience is with the form), comparison structure (whether the target contrast is adjacent, cross-panel, or cross-chart), and headroom (does it survive when the data doubles?). The optimum within the set shifts with audience and setting: for a general audience a pie may beat parallel coordinates that would suit a doctoral reader — and vice versa.

Where it stops holding

Viable is not equivalent: the set has hard edges, and a candidate a precision class down is out whenever the question needs magnitudes (the mental-arithmetic test in the first leaf). When several questions coexist, split them first and enter each viable set separately, rather than compromising on one "all-purpose" chart. A novel chart enters the viable set only if interpretable: viable means readers extract the target answer without instruction — otherwise it is merely novel.

Applying it

• List 2–3 candidate charts for the question and score them on precision, familiarity, comparison structure, and headroom before finalizing, instead of shipping the first one that renders.
• When candidates tie, pick the one readers know best (learning cost is chronically underestimated); under growth pressure, pick the one with headroom.
• Verification: build both candidates, give each to a reader group answering the target question, compare time and error rate; a significant gap is the cost comparison delivering its verdict.

Related

• Same group: U2.17.1 A mismatched chart forces readers to compute the answer mentally · U2.17.3 Automatic recommendation reads data types, not analytical intent · U2.17.4 Choosing the chart first manufactures questions to fit it · U2.17.5 As data magnitude shifts, a once-fitting chart stops working
• Nearby: U2.01.3 The same data supports several tasks; draw a chart for each · U2.07.3 Stacked bars are usually the better substitute
• Search terms: chart alternatives · reading cost · graphical literacy · chart selection