Heatmap encodes values as color value of a cell in a row-column grid. Toussaint Loua’s 1873 crime tables; later Toussaint/Bertin and 1990s gene arrays.
In reports, ads and assignments it fits patterns across two categorical axes at once. Skip it when precise lookup of a number, or a rainbow that is not color-blind safe.
Principles
- Encoding — Heatmap encodes color value of a cell in a row-column grid.
- History — Toussaint Loua’s 1873 crime tables; later Toussaint/Bertin and 1990s gene arrays
- Fit — patterns across two categorical axes at once
- Shared baseline — The eye compares length and position more accurately than area.
- Audience — PowerPoint, Excel or Google Sheets and Meta, Google and retail print share the same geometry.
Misconceptions
- Pretty equals true — Decoration rarely persuades; it slows reading.
- 3-D helps — Perspective distortion wrecks comparison. Keep it flat.
- More series is richer — Do not stack stories. One Heatmap, one claim.
- Default palette is fine — Rainbow defaults fail color-blind viewers and grey print. Change Excel or Google Sheets defaults.
Why it matters
- Work — Work: patterns across two categorical axes at once. Readable in four minutes in PowerPoint.
- Ads — Ads: one number and one shape speak first in Meta, Google and retail print.
- School — School: for lab reports and poster boards, pick the form that answers the rubric.
- Craft — Craft: honour color value of a cell in a row-column grid and decoration falls away.
Heatmap: honour color value of a cell in a row-column grid, and switch when precise lookup of a number, or a rainbow that is not color-blind safe.