A contour plot (isopleth diagram) based on samples each occupying perfectly ordered tiles of an R2 surface. An "R2.5" plot. The X and Y cartesian primes of the plot may be different metrics, but the Value of each tile (the .5 Real dimension) is considered an "intensity".

For the plot to make sense Value must be ordinal, but X or Y may be cardinal if if the user is expected to understand them.

To emphasise "hot" as "more" on the value axis, the plot displays high intensity with the hottest color of the palette.

• http://www.senorjosh.com/heatmap.html