Taking time with visualisation
What it shows
A heat map uses a matrix layout with colour and shading to show the relationship between two categories of values. The values are presented along each axis and the corresponding cell is then colour-coded to represent the relationship between the two categories. A heatmap might be used to show the percentage of people surveyed in different countries (y axis) who voted for different favourite sports (x axis). The darker cells would indicate they were the most popular sports in each country.
How to read it
Look for the general hierarchy of shades from the darkest (highest values) through the mid range shades and to the light shades (lowest).
Things to beware
It is not always easy to determine the exact quantitative values represented by the colours, even if there is a key provided. Unless a value label is included in each cell, try not to expect to be able to judge precise values from the colour shades.