A heat map (or heatmap) is a graphical representation of data where the individual values contained in a matrix are represented as colors. It is a bit like looking a data table from above. It is really useful to display a general view of numerical data, not to
extract specific data point. It is quite straight forward to make a heat map, as shown on the examples below. However be careful to understand the underlying mechanisms. You will probably need to normalise your matrix, choose a relevant colour
palette, use cluster analysis and thus permute the rows and the columns of the matrix to place similar values near each other according to the clustering.
Native Heatmap Function
Levelplot function from lattice