Use an aligned bar chart instead of a pie chart for proportion comparison

Use an aligned bar chart when readers must judge how large one quantitative value is relative to another. For example, show the marked values as bars on a common linear scale instead of pie slices judged by angle.

An aligned bar chart puts both values on one scale, so the eye compares positions directly. A pie chart asks readers to judge angles, which is less precise for quick percentage estimation between marked values.

Mechanism: Common-scale bars reduce the comparison to a shared positional read. Pie slices require angle judgment, which adds more perceptual error in exact proportion tasks.

Evidence: In the collated ranking, the three aligned bar conditions all scored above the pie condition for accuracy, and each was significantly better than the pie condition under the reported 95% bootstrap test for proportional judgments (Zeng & Battle, 2023; Heer & Bostock, 2010).

• User Goal: Compare two marked quantitative values as a percentage or relative size.
• Task: Make a quick but accurate proportional judgment.
• Data: Quantitative values that could be shown either as bars or as slices.
• Chart Setting: A static single-view chart on a standard display, with an active choice between an aligned bar chart and a pie chart.
• Audience: Readers making quick visual judgments.
• Success Criterion: Higher accuracy in the proportion comparison.

Break it when: Exact proportional comparison between marked values is not the job of the chart. Why: This evidence only supports quick accuracy for that specific comparison task.

Sacrifice: You give up the angle-based pie form.
Risk: Switching to bars without a common scale can erase the tested advantage.
Mitigation: Keep the chosen bar chart aligned to one linear baseline.

Mistake: Replacing the pie with a bar chart that still makes readers compare bars from unequal starting points. Why it fails: The supported advantage comes from aligned bar positions, not from any bar arrangement.

Failure Sign: The current design asks readers to compare slice angles for the main judgment.
Quick Check: Make an aligned bar version of the same values and see whether the main comparison moves onto one shared axis.
Stronger Test: Ask a reviewer to estimate the smaller value as a percent of the larger in both versions and keep the aligned bar chart if the answers are more consistent.

• Replace the pie slices with bars on a common linear scale.
• Mark the compared values directly on the bar chart.
• If the first bar version still uses unequal baselines, rework it into an aligned bar layout before finalizing the switch.