Lesson 16 of 20
Pie charts — why there is exactly one in 62 panels
Fig. 5F · circles whose radius carries the proportion
Of the paper's 62 panels, exactly one is a pie chart. Fig. 5F. Against 14 panels of bars, that is a stark ratio.
The reason is already in lesson 2. A pie chart loads its values onto angle (5th) and area (6th). That is two or three rungs below a bar's length (2nd).
Pies are especially bad in three situations.
| Situation | Why | Instead |
|---|---|---|
| more than five slices | the small slices become indistinguishable | sorted horizontal bars |
| comparing two pies | slices in different pies start at different angles, so they cannot be compared | grouped or stacked bars |
| values that are close | a 3% gap is 11 degrees. Invisible | bars |
There is one case where a pie is fine — two slices, and the reader only needs “about half or not”. But in that case a single line of text is better still.
In the pie on the left, are you sure whether A or B is larger? In the bars on the right there is nothing to think about.
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The values are 1, 2, 3, 4. On the left the radius is proportional, on the right the area is. The right one is correct.
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🤔 Check yourself
What is the fundamental reason a pie chart is worse than a bar chart?
Answer In the Cleveland–McGill ranking angle is 5th and area 6th, while length is 2nd. Same data, different reading accuracy.
🤔 Check yourself
When a circle's size encodes a value, why should the radius be √value?
Answer Make r proportional to the value and the area scales with its square. A fourfold value looks sixteenfold. Fig. 5F has exactly this problem.