Assign a closed shape to the focal class in cluttered scatterplots

Assign a closed shape to the symbol class readers must pick out or follow in a dense mixed-symbol scatterplot. For example, use a circle, square, or triangle for the target class rather than a plus, x, or asterisk when readers must identify that class amid distractors or read a linear relationship from the same plot.

Closed shapes were processed faster and more accurately than open shapes when they served as targets. That advantage carries into cluttered mixed-symbol plots when readers must focus on one class and ignore others.

Mechanism: A closed focal shape is selected more efficiently, so readers spend less time resolving which marks belong to the target class before responding.

Evidence: In both perceptual tasks, closed targets produced faster and more accurate responses than open targets, and in single-plot linear-relationship judgments closed targets were read more quickly while same-feature distractors created stronger interference than different-feature distractors (Burlinson et al., 2018).

Notes: The paper reports that distractor category matters too, with same-category distractors interfering more than different-category distractors.

• User Goal: Pick out one symbol class quickly or follow one class through a mixed display.
• Task: Identify a target category while ignoring distractor symbols, or judge a linear relationship for one class inside a mixed plot.
• Data: A focal categorical class appears among other symbol classes in a cluttered display.
• Chart Setting: A single scatterplot contains heterogeneous symbols and the reader must attend to one class over the others.
• Success Criterion: Faster and more accurate responses to the focal class.

• Break it when: Symbols are shown alone in homogeneous side-by-side plots rather than intermixed in one display. Why: The study did not find a consistent open-versus-closed response-time advantage in separate-plot displays.
• Break it when: The task is average-value judgment rather than target identification or linear relationship. Why: The paper did not show reliable target-feature effects there.

Sacrifice: You constrain which symbol can represent the focal class.
Risk: A closed focal shape can still be hard to read if distractors are also closed.
Mitigation: Pair the closed focal shape with distractors from the open category when possible.

Mistake: Give the focal class an open shape, or pair a closed focal shape with same-category distractors, in a dense mixed plot. Why it fails: Open targets were slower overall, and same-feature distractors especially interfered with closed targets.

Failure Sign: Readers are slower or less accurate when picking out the focal class than expected for the same data and density.
Quick Check: Compare otherwise identical dense plots that differ only in whether the focal class uses a closed or open shape.
Stronger Test: Repeat the comparison with same-category and different-category distractors to see whether the focal class still suffers interference.

• Switch the focal class from an open symbol to a closed symbol.
• If the focal class already uses a closed symbol, move distractor classes to the open category when possible.
• Apply this change first in dense mixed plots where readers must track one class through distractors.