Use a line chart to emphasize a trend across ordered points
For reading change across ordered data points, use a line chart instead of a bar chart on ordered values to improve readability and mitigate isolated point-by-point reading for viewers relying on common graph conventions.
- purpose:select
- basis:empirical
- task:trend
- chart:line:use
- chart:bar:avoid
- quality:readability:use
- lever:chart-family
- reading-mode:overview
advice
Choose lines for trends
Choose a line chart when the main message is how values change across an ordered sequence. For example, connect successive measurements with a line when you want viewers to read the overall rise or fall rather than treat each point as a separate category.
reason
Why lines fit trends
Readers expect a line to express a connected progression. That expectation helps them extract the overall direction of change instead of focusing only on isolated point-by-point differences.
Mechanism: A line chart organizes the points into one continuous structure, which guides attention toward trend across the sequence.
Evidence: The paper explains that readers implicitly treat line graphs as highlighting trends across multiple data points and contrasts this with the comparison-focused reading encouraged by bar graphs (Zacks & Franconeri, 2020).
context
Use when the message is overall change
- User Goal: Understand rise, fall, or other overall pattern across an ordered sequence.
- Task: Trend reading.
- Data: Multiple values arranged in meaningful order.
- Chart Setting: A single series or comparable ordered points on one common scale.
- Audience: Readers using common graph conventions.
- Success Criterion: Readers describe the direction or pattern of change.
exceptions
Do not use when the task is comparing discrete groups
Break it when: The main task is comparing named categories or separate groups. Why: Readers use bar charts to interpret comparisons between individual data points.
costs
Gain trend structure but lose category emphasis
Sacrifice: Individual point-by-point category comparisons become less prominent. Risk: If the x positions are not meaningfully ordered, the line suggests continuity that is not really there. Mitigation: Use bars when the x axis is categorical rather than ordered.
mistakes
Avoid flattening a trend into separate categories
Mistake: Using bars when the message is the trend across the series. Why it fails: Readers focus on isolated values instead of the overall change.
check
Compare the trend cue directly
Failure Sign: Readers report only point-by-point differences and miss the overall direction. Quick Check: Show a bar and line version to a few viewers and ask what pattern they see first. Stronger Test: Keep the version that elicits trend descriptions rather than isolated comparison statements.
fix
Replace category marks with a connected sequence
- Replace separate bars with a connected line.
- Keep the order explicit on the x axis.
- Remove bar-specific emphasis when the trend is the message.