I am working my way through The R Graphics Cookbook and ran into this set of code:
library(gcookbook)
library(ggplot2)
p <- ggplot(faithful, aes(x = eruptions, y = waiting)) +
geom_point() +
stat_density2d(aes(alpha=..density.., fill=..density..), geom="tile", contour=FALSE)
It runs fine, but I don't understand what the .. before and after density is referring to. I can't seem to find it mentioned in the book either.