Use two closed shapes for trend judgments in mixed scatterplots

Use two closed shapes when one scatterplot must support trend judgments across two shape-coded groups. For example, replace an open-open pair such as asterisk plus cross with a closed-closed pair such as triangle plus square when both groups occupy the same mixed point cloud.

Closed shapes are separated more efficiently than open line-based shapes when both groups share one dense field of points. That makes it easier to decide which group forms the clearer linear pattern.

Mechanism: Closed shapes reduce interference during mixed-symbol trend reading, so readers can identify the more linear group faster and with fewer mistakes.

Evidence: In the collated results, the closed-closed triangle-square pair ranked first for both accuracy and time on the correlate task, while the open-open asterisk-cross pair ranked last; the reported significant pairwise difference was closed-closed over open-open. The source paper also reports faster and more accurate processing for closed than open shapes and a closed-target advantage in single-plot linear-relationship judgments (Zeng & Battle, 2023; Burlinson et al., 2018).

Notes: The advantage depends on task and on whether both groups are mixed into one plot.

• User Goal: Decide which of two groups shows the stronger linear relationship.
• Task: Trend judgment across two shape-coded groups.
• Data: Two quantitative axes with one categorical grouping mapped to shape.
• Chart Setting: One scatterplot contains both groups at once, and the point field is visually dense or cluttered.
• Audience: Readers making quick visual analytic judgments.
• Success Criterion: Higher trend-reading accuracy with shorter response time.

Break it when: The groups are shown in separate homogeneous plots, or the primary task is average-value judgment rather than trend judgment. Why: The study did not find a reliable open/closed advantage in side-by-side homogeneous displays, and the average-value task showed no significant shape-category effect.

Sacrifice: The rule is task-specific rather than a universal ranking for every scatterplot task.
Risk: The benefit can weaken or change with difficulty and with the distractor category paired against the target.
Mitigation: Validate the closed-closed mapping on the actual mixed-plot trend task instead of assuming it will transfer to other tasks.

Mistake: Keep two open shapes in the same mixed scatterplot when the chart’s job is trend judgment. Why it fails: The open-open pair was the worst trend condition, and open shapes were slower and less accurate to process than closed shapes.

Failure Sign: Reviewers hesitate or disagree about which group forms the clearer line.
Quick Check: If both legend symbols are open line-segment shapes in one mixed scatterplot, flag the chart for revision.
Stronger Test: Compare the current mapping against a triangle-square version on the same data with a short timed trend-judgment check.

• Replace the two open symbols with two closed symbols.
• Start with a triangle and square if you need a direct source-tested pair.
• If your chart already uses separate homogeneous plots, do not spend revision time on open-versus-closed pairing for this task.