Prefer color-saturation over area for a secondary quantitative field during exact value reading

Encode the secondary quantitative field with color-saturation when the main quantitative field must be read from position. For example, keep the main quantitative field on x or y and map the other quantitative field to color-saturation instead of varying mark area when readers must read exact values or compare two values.

Varying mark area acts as a distractor when the reader is trying to decode a different quantitative field from position. Color-saturation preserved better value-reading performance for the positioned field in these exact-reading tasks.

Mechanism: Color-saturation leaves the positioned readout visually stable, while changing mark area makes the positioned field harder to decode quickly and accurately.

Evidence: In controlled tests of trivariate point-based views, the designs with the main quantitative field on position and the secondary quantitative field on color-saturation outperformed the corresponding area-based designs for retrieve-value and compare-values tasks in both accuracy and completion time (Zeng & Battle, 2023; Kim & Heer, 2018).

Notes: This rule is about protecting the readability of the primary positioned field, not about maximizing readability of the secondary field.

• User Goal: Read exact values or compare two individual values of a main quantitative field.
• Task: Exact lookup or pairwise value comparison.
• Data: One categorical field and two quantitative fields.
• Chart Setting: A point-based multivariate view already uses position for the main quantitative field and still needs to show a second quantitative field.
• Success Criterion: Lower error and faster responses on individual-value tasks.

Break it when: The main task is to find group maxima or compare group averages. Why: The value-task advantage of color-saturation over area does not define the best choice for those summary tasks, where size-based encodings performed well.

Sacrifice: You give up the size-based summary cue that can help on some summary judgments. Risk: Applying this rule to summary tasks can miss the benefit that area-based encodings showed there. Mitigation: Use this rule only when exact reading of the positioned field is the main requirement.

Mistake: Encode the secondary quantitative field with varying mark area while expecting fast, exact reading of the primary positioned field. Why it fails: The changing area interferes with decoding the positioned field.

Failure Sign: Readers hesitate or misread the main positioned value when mark sizes vary. Quick Check: Make two versions of the same chart, changing only the secondary quantitative field from area to color-saturation, and compare one lookup question and one pairwise comparison question. Stronger Test: Measure answer time and error on a small set of representative exact-reading prompts.

• Remap the secondary quantitative field from area to color-saturation.
• Keep the main quantitative field on x or y.
• Revisit the encoding only if the task shifts from exact value reading to group-level summaries.