Heatmap

A heatmap is a chart laid out as a grid of cells in which colour intensity encodes the value of each cell, letting patterns across two dimensions jump out at a glance.

Instead of using height or length, a heatmap uses colour to carry the number. Each cell is shaded along a colour scale — light to dark, or one hue to another — so that hot spots and cold spots become visible immediately. It is built for showing how a value varies across a two-way table, where a grid of plain numbers would be hard to scan.

How a heatmap works in a chart

The rows and columns of the grid are usually categories or time periods, and each cell sits at the crossing of one row and one column. The cell's colour is mapped to its value through a colour scale, and a colour key (a form of legend) tells the reader which shade means which number. Because the eye reads colour roughly rather than precisely, heatmaps excel at revealing where the highs and lows cluster but are weaker for reading exact figures.

A concrete example

Imagine a grid with the seven days of the week as columns and 24 hours as rows, each cell holding the number of visits to a website in that hour. Cells with few visits are pale; busy cells are dark. Even without reading a single number, a dark band across the late-morning rows on weekdays instantly shows when traffic peaks. A grid of 168 plain numbers would hide that pattern; the colour makes it obvious.

Related terms

A heatmap relies on a colour scale and a legend to decode its shading, and each cell is effectively one data point. For exact values readers often add data labels inside the cells.