ggplot2: Quick Heatmap Plotting

A post on FlowingData blog demonstrated how to quickly make a heatmap below using R base graphics.

This post shows how to achieve a very similar result using ggplot2.

Data Import

FlowingData used last season’s NBA basketball statistics provided by databasebasketball.com, and the csv-file with the data can be downloaded directly from its website.

> nba <- read.csv("http://datasets.flowingdata.com/ppg2008.csv")

The players are ordered by points scored, and the Name variable converted to a factor that ensures proper sorting of the plot.

> nba$Name <- with(nba, reorder(Name, PTS))

Whilst FlowingData uses heatmap function in the stats-package that requires the plotted values to be in matrix format, ggplot2 operates with dataframes. For ease of processing, the dataframe is converted from wide format to a long format.

The game statistics have very different ranges, so to make them comparable all the individual statistics are rescaled.

> library(ggplot2)

> nba.m <- melt(nba)
> nba.m <- ddply(nba.m, .(variable), transform,
+     rescale = rescale(value))

Plotting

There is no specific heatmap plotting function in ggplot2, but combining geom_tile with a smooth gradient fill does the job very well.

> (p <- ggplot(nba.m, aes(variable, Name)) + geom_tile(aes(fill = rescale),
+     colour = "white") + scale_fill_gradient(low = "white",
+     high = "steelblue"))

A few finishing touches to the formatting, and the heatmap plot is ready for presentation.

> base_size <- 9
> p + theme_grey(base_size = base_size) + labs(x = "",
+     y = "") + scale_x_discrete(expand = c(0, 0)) +
+     scale_y_discrete(expand = c(0, 0)) + opts(legend.position = "none",
+     axis.ticks = theme_blank(), axis.text.x = theme_text(size = base_size *
+         0.8, angle = 330, hjust = 0, colour = "grey50"))

Rescaling Update

In preparing the data for the above plot all the variables were rescaled so that they were between 0 and 1.

Jim rightly pointed out in the comments (and I did not initally get it) that the heatmap-function uses a different scaling method and therefore the plots are not identical. Below is an updated version of the heatmap which looks much more similar to the original.

> nba.s <- ddply(nba.m, .(variable), transform,
+     rescale = scale(value))

> last_plot() %+% nba.s