Heatmap

A heatmap lays out the cells of a cross tabulation as a grid and encodes the cell value through colour intensity. Without a value column each cell shows the frequency of the combination; with one it shows the mean or the sum within it.

• Both axes categorical — the category order is free and determines which patterns stand out
• Sequential colour scale — light to dark for magnitudes; only a diverging scale with a neutral midpoint suits deviations in both directions
• Empty cells — combinations without data must be visibly distinct from "value zero"

Typical uses: defect counts by shift × line, mean processing time by site × order type. Its strength is scanning many cells at once — where reading a table of numbers would be too slow.