Use scatterplots instead of pie charts for anomaly detection
For anomaly detection in small static two-dimensional displays, use a scatterplot instead of a pie chart on tabular data to improve insight and accuracy and mitigate missed outliers for readers scanning for abnormal values.
- purpose:select
- basis:empirical
- chart:scatter:use
- chart:pie-donut:avoid
- data:tabular
- quality:insight:use
- lever:chart-family
- reading-mode:overview
advice
Choose scatter over pie
Use a scatterplot when the main question is whether any displayed value looks abnormal. For example, replace a pie chart with a scatterplot when readers must spot an outlying point rather than compare slices.
reason
Why scatter works better here
A scatterplot exposes individual points and makes unusual positions easier to notice. A pie chart organizes the display around slices, which makes abnormal observations harder to isolate.
Mechanism: Point positions in a scatterplot let readers scan for values that break the visible pattern, while a pie chart centers attention on wedge sizes instead of outlying cases.
Evidence: In the experiment, scatterplots ranked above pie charts for anomaly detection in accuracy and user preference, and the later review summarizes this study as recommending scatterplots for find-anomalies tasks. (Saket et al., 2019; Zeng & Battle, 2023)
Notes: The paper’s own summary recommendation for this task was to use scatterplots.
context
Use when the task is spotting abnormal values
- User Goal: Identify whether one or more displayed values are abnormal.
- Task: Find anomalies.
- Data: Tabular data shown in a small display with 5-34 marks.
- Chart Setting: Static two-dimensional chart.
- Success Criterion: Higher anomaly-detection accuracy with a chart people prefer to use.
exceptions
Do not use when the task changes to part comparison or exact lookup
Break it when: the task changes from spotting abnormal points to reading exact values or making other non-anomaly judgments such as retrieve-value tasks. Why: the study did not show scatterplots as the strongest option for exact lookup, and other chart types performed better there.
costs
Costs of switching from pie to scatter
Sacrifice: You give up the pie chart’s slice-based presentation. Risk: A scatterplot is not automatically the best choice for every task on the same data. Mitigation: Use the scatterplot when finding anomalies is the main question, not just one possible secondary read.
mistakes
Common failure mode
Mistake: Keep a pie chart for anomaly detection because it already separates categories into slices. Why it fails: slice comparison does not make abnormal observations as visible as point positions do.
check
Check the task against the chart choice
Failure Sign: Reviewers must find an abnormal value from a pie chart. Quick Check: Render the same data as both a scatterplot and a pie chart and ask which version makes the abnormal case easier to spot. Stronger Test: Run one anomaly question on both versions and compare which chart produces fewer missed anomalies.
fix
Fix the chart choice
- Replace the pie chart with a scatterplot when the goal is to find abnormal values.
- Redraw the same observations as individual points so unusual positions can stand out.
- If the pie chart must stay for another purpose, add a separate scatterplot for anomaly finding.