Use box plots instead of line graphs for interval spread comparisons

Choose a box plot when readers must compare how spread out values are within each time interval. For example, use one box plot per month instead of a line graph when the question is which interval is most spread out from its average.

Spread comparison depends on seeing interval-level variation, not tracing a raw sequence of points. Box plots expose summary distribution structure directly, while line graphs force readers to infer spread from many individual values.

Mechanism: The box, whiskers, and central summary reduce the interval to a directly comparable distribution cue, so readers do less visual estimation of variability.

Evidence: In the time-series experiment, the box plot was the top-performing design for the spread task and outperformed the plain line graph, and the review summarizes the same paper as showing that different chart types support different summary tasks differently. (Albers et al., 2014; Zeng & Battle, 2023)

• User Goal: Compare dispersion across repeated time intervals.
• Task: Interval spread comparison.
• Data: Ordered time series partitioned into repeated intervals such as months.
• Chart Setting: Readers do not need to inspect every raw point while making the spread decision.
• Success Criterion: Higher answer accuracy on interval spread questions.

Break it when: Readers must inspect raw point values or count outliers within each interval. Why: the box plot replaces the raw series, and the study did not use it for the outlier-count task.

Sacrifice: Direct visibility of every raw value in the series. Risk: Tasks that depend on local features or anomaly counts can become harder or unsupported. Mitigation: Use a different view when the task depends on raw points instead of spread summaries.

Mistake: Keep a line graph for a spread comparison because it preserves every point. Why it fails: Readers must infer dispersion from many points, and accuracy is lower than with a box plot.

Failure Sign: Reviewers trace the whole line to estimate which interval varies most. Quick Check: Show the same spread question once with a line graph and once with interval box plots. Stronger Test: Keep the box plot if it yields more accurate answers to a forced-choice question about which interval is most spread out.

• Replace the line graph with one box plot per target interval.
• Keep the boxes in the same time order as the original series.
• If the real question is range rather than spread, switch to explicit interval minima and maxima instead of a line graph.