Darker cells mean a higher score for this topic on that metric.
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Quick answers
When should I pick Heatmap?
When the story is: patterns across two categorical axes at once.
When should I avoid Heatmap?
Skip it in cases like: precise lookup of a number, or a rainbow that is not color-blind safe.
Who invented or popularised it?
Toussaint Loua’s 1873 crime tables; later Toussaint/Bertin and 1990s gene arrays
What does it encode?
color value of a cell in a row-column grid
Is it OK in ads?
Yes if one claim is visible at a glance in Meta, Google and retail print; otherwise link to a full figure.
Will a teacher accept it?
Yes for lab reports and poster boards if axes, units and a source line are present.
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 — The core mark of Heatmap is 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.