14 Plotting Heatmap

Heatmap is useful when you want to display some aggregated values (frequency, average of a numeric column) against two categorical columns.

library(ggplot2)
data(diamonds)
d.agg = aggregate(price ~ color + cut, diamonds, mean)
ggplot(d.agg, aes(color, cut)) + 
    geom_tile(aes(fill = price)) + 
    scale_fill_gradient(low="lightblue", high="steelblue")