Use separate displays instead of co-plotting two series for average-position judgments

Show each data series in its own display instead of co-plotting two series when readers must judge each series’ average vertical position after brief viewing. For example, separate two lines, two bar series, or a line-plus-bar pair into different displays rather than placing both series in one shared frame when viewers must reproduce one target series’ average height after a short delay.

A second series in the same display shifts the remembered average of the target series toward the non-target series. Splitting the series removes that specific cross-series pull, which otherwise exaggerates or reduces the baseline bias depending on whether the non-target series sits above or below the target.

Mechanism: Co-plotted series act like anchors on each other, so the irrelevant series changes where viewers remember the target series’ average position to be.

Evidence: In compound displays with two lines, two bar series, or a line-plus-bar pair, target average-position estimates were pulled toward the irrelevant series, and the authors advise avoiding plotting two series in the same display when trying to avoid this effect (https://doi.org/10.1109/TVCG.2019.2934400">Xiong et al., 2020).

Notes: Changing one of the series from a line to bars does not remove the pull; the effect generalized across same-type and mixed-type pairs.

• User Goal: Read each of two series separately and reproduce each series’ average vertical position accurately.
• Task: View a target series briefly, then report its average level from memory.
• Data: Two quantitative series.
• Chart Setting: A design choice between one shared display and separate displays for the two series.
• Audience: Readers making memory-based judgments of one cued target series at a time.
• Success Criterion: Smaller cross-series distortion in reproduced average positions.

Break it when: The display contains only one series, or the task is not a delayed average-position report for one target series. Why: The paper establishes perceptual pull only for two-series displays in the tested average-position task.

Sacrifice: Readers no longer see both series in one shared frame. Risk: Splitting the series does not remove the baseline bias of the chosen chart family, because single lines were still underestimated and single bars were still overestimated. Mitigation: After separating the series, test each display for the remaining line or bar bias.

Mistake: Keep both series in one display but change one series to a different mark type and expect the interference to disappear. Why it fails: Line-plus-bar displays still showed perceptual pull, not a clean escape from it.

Failure Sign: Estimates for a target series shift toward the other series when both appear in the same display. Quick Check: Compare a shared two-series display against a version where each series is shown in its own display, using brief viewing, a short delay, and average-position reproduction. Stronger Test: Run the comparison for each pairing you might use, including two lines, two bar series, and a line-plus-bar pair.

• Remove the non-target series from the display used for the average-position readout.
• Put each of the two series in its own display instead of co-plotting both in one frame.
• Do not use a line-plus-bar combination as a workaround for a two-line or two-bar shared display.