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.
Parts
- Mark — The core mark of Heatmap is color value of a cell in a row-column grid.
- Scale — The scale must show zero (or a declared break) and units.
- Axis — Axis titles name the variable, not a mood.
- Legend — A legend is weaker than a direct label. Write beside the mark when you can.
- Label — Numbers are optional. If comparison is the job, label the ends only.
- Grid — Keep the grid faint. Data should own the ink.
- Annotation — One annotation sentence. Date the source.
- Frame — A thin frame. Never a 3-D box.
Variants
- Matrix shading — Variant 1 of Heatmap. Same encoding, different layout.
- Choropleth cousin — Variant 2 of Heatmap. Same encoding, different layout.
- Intensity grid — Variant 3 of Heatmap. Same encoding, different layout.
- Heatmap 4 — Variant 4 of Heatmap. Same encoding, different layout.
Labels
- Axis — On Heatmap, make Mark readable.
- Legend — On Heatmap, make Scale readable.
- Label — On Heatmap, make Axis readable.
- Grid — On Heatmap, make Legend readable.
- Annotation — On Heatmap, make Label readable.
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.