Use line charts to convey continuous trend

Choose a line chart when the main job is to show continuous change across an ordered sequence. For example, connect the observations with a line for trend displays and avoid bars when you want readers to read rise, fall, and shape rather than separate amounts.

A connected line changes what readers think the data is about. The same vertical positions can be read as separate quantities in bars or as a continuous trajectory in a line, so the chart family itself guides interpretation.

Mechanism: Connecting points creates an affordance for continuity and trend, while bars emphasize individual comparisons between discrete values.

Evidence: The paper summarizes evidence that people describe the same values differently in bars and lines: bars elicit discrete comparisons, while lines elicit trend descriptions. It uses this result to argue that chart choice should follow the intended interpretation, not only position precision (https://doi.org/10.1109/VIS47514.2020.00048">Bertini et al., 2020).

• User Goal: Explain change, direction, or overall shape across a sequence.
• Task: Trend reading rather than point-by-point comparison.
• Data: Sequential observations with a meaningful order and an implied continuity.
• Chart Setting: You are choosing between a bar chart and a line chart for the same values.
• Audience: Readers who need the chart to cue a trend interpretation.
• Success Criterion: Readers describe the series in terms of rise, fall, slope, or overall shape.

Break it when: The main job is to compare separate values as independent observations. Why: The line introduces a continuity cue that can shift attention away from discrete comparisons.

Sacrifice: You give up some emphasis on separate item-by-item comparison. Risk: Readers may infer continuity where the data should be treated as distinct observations. Mitigation: Switch to bars when the message is comparison between separate values rather than trend.

Mistake: Adding a connecting line when the values should be read as separate comparisons. Why it fails: The connection itself suggests continuity and trend.

Failure Sign: Reviewers talk about individual amounts instead of the overall trajectory. Quick Check: Render the same data as both bars and a line, then ask a reviewer for a one-sentence description. Stronger Test: Keep the version that makes reviewers describe the intended trend without prompting.

• Replace bars with a connected line across the ordered observations.
• Keep the ordered sequence visible so the line reads as one trajectory.
• Remove the line and switch to bars if reviewers keep treating the sequence as separate comparisons.