Just a short post to celebrate that I learned today how incredibly easy it is to make a heatmap of correlations with ggplot2 (and reshape2, of course).

data(attitude)
library(ggplot2)
library(reshape2)
qplot(x=Var1, y=Var2, data=melt(cor(attitude)), fill=value, geom="tile")

So, what is going on in that short passage? cor makes a correlation matrix with all the pairwise correlations between variables (twice; plus a diagonal of ones). melt takes the matrix and creates a data frame in long form, each row consisting of id variables Var1 and Var2 and a single value. We then plot with the tile geometry, mapping the indicator variables to rows and columns, and value (i.e. correlations) to the fill colour.