Encode the primary quantitative field with position for exact value tasks

Put the quantitative field people must read or compare exactly on x or y. For example, in a trivariate point plot, map the primary quantity to a positional axis and leave size or color for a secondary quantity or a category instead of putting the primary quantity on size or color.

Exact reading tasks depend on a fast, stable visual lookup. Position made the primary value easier to decode than retinal channels when participants had to read a marked point or compare two marked points.

Mechanism: Position supports more precise value lookup and pairwise comparison than size or color when readers must inspect individual records.

Evidence: Across value tasks, encodings that placed the primary quantitative field on x or y ranked higher overall than encodings that placed it on size or color (Kim & Heer, 2018).

• User Goal: Read a value from a marked point or decide which of two points is larger or smaller.
• Task: Exact lookup or exact pairwise comparison of individual records.
• Data: A point-based view with one primary quantitative field and at least one additional field competing for encoding channels.
• Chart Setting: A multivariate point plot where x and y are available for assignment.
• Success Criterion: Lower error and faster completion on individual-value questions.

Break it when: The task is to compare group averages or identify which category contains the maximum. Why: In the study, size encoding of the primary quantity performed well for these summary tasks, so the value-task advantage of position does not transfer cleanly.

Sacrifice: You give positional priority to one quantity rather than another field. Risk: A less important field may be pushed to a weaker or more interfering channel. Mitigation: Keep the primary task field on position and move the less critical field to a non-positional channel.

Mistake: Put the primary quantitative field on size or color because another field already occupies position. Why it fails: The chart becomes worse for exact value reading and comparison, which were the tasks where position was most effective.

Failure Sign: Readers hesitate or make mistakes on marked-point value questions. Quick Check: Compare the current chart against a version where the primary quantitative field is moved to x or y. Stronger Test: Ask one read-value question and one compare-values question on both versions and keep the lower-error, faster version.

• Move the primary quantitative field to x or y.
• Reassign the displaced field to color, size, or faceting.
• If both positional axes are occupied, rebuild the point plot so the task-critical quantity gets a positional axis.