Use no more than four shapes in grouped scatterplots
For grouped scatterplots, prefer shape encoding on point marks to improve accessibility and mitigate color-only grouping, and avoid large shape sets that create visual clutter for readers with color-vision deficiency.
- purpose:refine
- basis:heuristic
- chart:scatter
- quality:accessibility
- lever:encoding
- channel:shape:use
- group-cardinality:few
- needs:color-vision-deficiency
advice
Replace color-only grouping with a small shape set
Use distinct point shapes in a scatterplot when color alone separates groups. For example, replace some circles with triangles, rectangles, crosses, or stars, but stop at three or four shapes instead of giving every group a different marker.
reason
Why a small shape set works
Shapes provide a noncolor way to separate groups, but too many shape types overwhelm the display. A small, repeated set preserves group identity without turning the plot into visual confetti.
Mechanism: Shape differences survive color confusion, while limiting the number of shapes preserves a readable point field.
Evidence: The article recommends using different shapes in scatterplots instead of just circles and warns that using all those shapes quickly looks like confetti, advising a limit of three or four. (Muth, 2020)
context
Use when a scatterplot has a few groups
- User Goal: Distinguish grouped points in a scatterplot.
- Task: Compare or identify categories in a point cloud.
- Data: A few groups encoded on the same scatterplot.
- Chart Setting: A scatterplot where color would otherwise be the only group marker.
- Audience: Readers who may include people with color-vision deficiency.
- Success Criterion: Groups remain distinguishable without the plot becoming cluttered.
exceptions
When shape encoding breaks down
Break it when: The scatterplot needs more than four distinct group markers. Why: The article warns that larger shape sets quickly make the plot look like confetti.
costs
Costs of adding point shapes
Sacrifice: You give up some visual uniformity in the point field. Risk: Too many shapes create clutter and slow scanning. Mitigation: Keep the shape set small and reuse only a few clearly distinct markers.
mistakes
Common scatterplot failure
Mistake: Encoding all groups with circles and asking readers to rely on color alone, or adding too many different shapes. Why it fails: The first choice depends entirely on color, and the second makes the plot visually noisy.
check
How to verify point shapes
Failure Sign: The plot either collapses into same-looking colored circles or turns into a field of many unrelated marker types. Quick Check: Count the distinct shapes in the plot and keep the total at four or fewer. Stronger Test: View the plot with color differences reduced and check whether groups still separate by marker shape.
fix
What to change
- Replace some circle-only groups with clearly different marker shapes.
- Limit the distinct marker set to three or four shapes.
- Merge or split groups differently if the plot would need more shapes than that.