Use a diverging color scheme around a meaningful center
For overview comparison of regional deviations around a meaningful center, use a diverging color scheme on a choropleth to improve readability and mitigate hiding one side of the scale for readers scanning both extremes.
- purpose:refine
- basis:heuristic
- chart:choropleth
- quality:readability
- lever:encoding
- channel:color-lightness:use
- operator:difference
- reading-mode:overview
advice
Diverging palette choice
Use a diverging color scheme when the mapped values have a meaningful middle and both ends of the scale deserve attention. For example, color values below and above a central reference with different hues and keep the center value in the middle of the scale.
reason
Why two-sided scales reveal two-sided differences
A center-based palette makes above-versus-below differences visible at a glance.
Mechanism: A diverging scheme separates the two sides of a meaningful midpoint while still showing how far regions are from that center. Readers can locate both extremes instead of only the high end.
Evidence: The post recommends diverging schemes when attention should go to both extremes, such as differences between competing sides (Muth, 2018).
context
Use when the midpoint matters
- User Goal: Show which regions are on either side of a reference value.
- Task: Compare negative versus positive or lower versus higher deviations.
- Data: One regional measure with a meaningful center.
- Chart Setting: A choropleth is already chosen.
- Audience: Readers scanning both ends of the distribution.
- Success Criterion: Both sides of the center are easy to distinguish.
exceptions
Do not use when the data is one-directional or unordered
Break it when: Only the high end needs emphasis, or the categories are unordered. Why: The post recommends sequential schemes for one-direction magnitude and qualitative schemes for unordered categories.
costs
Costs of a two-sided palette
Sacrifice: You reduce the emphasis on only one end of the scale. Risk: The midpoint can disappear if it is not clearly set apart. Mitigation: Make the middle color the lightest and keep the extremes darker.
mistakes
Common centerless divergence
Mistake: Use a diverging palette without a meaningful center value. Why it fails: The colors imply a split in the data that the values do not actually have.
check
Check whether the map really needs two sides
Failure Sign: One side of the scale matters, but the palette still splits the map into two hues. Quick Check: Confirm that the legend shows an explicit center value and that the story depends on both ends of the scale. Stronger Test: If removing the center would not change the message, switch back to a sequential scheme.
fix
Edit the palette around the midpoint
- Set an explicit center value for the scale.
- Assign one hue to values below the center and another hue to values above it.
- Make the center color the lightest point in the palette.
- Darken both extremes so the ends of the scale read clearly.