Place the primary quantitative field on the x-axis in row-faceted point plots

Place the primary quantitative field on the x-axis when categories are split into row facets and readers must read values or find maxima. For example, in a row-faceted point plot, swap the primary and secondary quantitative fields so the main field uses the common horizontal axis rather than the y-axis.

The horizontal axis stays aligned across stacked panels, while the y-axis requires harder cross-panel lookups and can force the reader to scroll to the bottom to see the needed scale. That made the x-assigned version faster or more accurate in the studied row-faceted tasks.

Mechanism: A shared horizontal scale supports easier panel-to-panel comparison than repeated vertical lookups in stacked facets.

Evidence: Within the row-faceted designs, the version with the primary quantitative field on x was faster for read-value and compare-values tasks and more accurate for find-maximum than the transposed version with the primary field on y (Zeng & Battle, 2023; Kim & Heer, 2018).

Notes: This finding was reported for row facets specifically.

• User Goal: Read exact values, compare individual values, or find the group containing the maximum value.
• Task: Lookup, pairwise comparison, or maximum search.
• Data: One categorical field split into row facets plus two quantitative fields.
• Chart Setting: A row-faceted point plot is already chosen and the main quantitative field can be assigned to either x or y.
• Success Criterion: Faster or more accurate reading across stacked panels.

Break it when: The layout is not row-faceted or the task is not one of the studied lookup or maximum tasks. Why: The reported advantage was specific to row facets and was not established as a general x-over-y rule for other layouts or tasks.

Sacrifice: The secondary quantitative field must take the y-axis. Risk: Leaving the primary field on y preserves the slower, harder cross-panel lookup pattern. Mitigation: Treat axis assignment as part of the facet design, not as a neutral swap.

Mistake: Keep the primary quantitative field on the y-axis in stacked row facets when readers must compare across panels. Why it fails: The repeated vertical lookups are less aligned and slower than reading from the shared horizontal axis.

Failure Sign: Readers must compare y positions across stacked panels or scroll to find the primary scale. Quick Check: Transpose the two quantitative axes and compare one read-value question and one maximum-search question. Stronger Test: Measure time and error for the x-assigned and y-assigned versions on a small set of row-faceted tasks.

• Move the primary quantitative field from y to x.
• Move the secondary quantitative field to y.
• Keep the category field on row facets while re-testing the same tasks.