Make independent y-axis scales explicit in small multiples

Make independent y-axis scales explicit whenever small-multiple panels do not share the same y-axis. For example, note in the chart description that the panel scales differ and use gridlines that look unusual enough to signal that the axes are not shared.

Independent scales change what equal visual height means across panels. Unless that change is clearly signaled, readers can mistake shape comparison for magnitude comparison.

Mechanism: Explicit scale signaling forces readers to re-check the axes before comparing panels and reduces false inferences from visually similar heights or slopes.

Evidence: The post warns that readers may assume shared y-axes across panels, advises avoiding independent scales when possible, and recommends explicit description text and unusual-looking gridlines when scales differ (Muth, 2024).

Notes: Independent panel scales are also presented as useful when magnitudes differ so much that shared scales hide the trend shape.

• User Goal: Show trend shapes despite large magnitude differences across series.
• Data: Small-multiple line chart panels that use different y-axis scales.
• Chart Setting: Independent panel scales with a risk that readers assume one shared axis.
• Success Criterion: Readers notice that scales differ before drawing cross-panel magnitude conclusions.

Break it when: Shared y-axis scales are possible without hiding the relevant trends. Why: The post recommends avoiding independent scales when possible.

Sacrifice: You give up direct cross-panel magnitude comparability. Risk: Readers may assume a shared scale and draw false insights. Mitigation: Prefer shared scales when possible; otherwise disclose the difference clearly on the chart.

Mistake: Use independent y-axis scales without warning readers. Why it fails: Readers may treat the panels as if they share one scale and misread the magnitudes.

Failure Sign: The panels look as if they share one y-axis when they do not. Quick Check: If the chart description does not explicitly say the scales differ and the gridlines look normal, readers may miss the scale change. Stronger Test: Ask whether a reader could infer a false cross-panel magnitude comparison from equal visual heights.

• Switch to shared y-axis scales if the chart can still show the needed trend differences.
• Add a chart-description note that the y-axis scales differ across panels.
• Use gridlines that do not look like standard shared-scale gridlines.