How to overlay density plots in R?

Question

I would like to overlay 2 density plots on the same device with R. How can I do that? I searched the web but I didn't find any obvious solution.

My idea would be to read data from a text file (columns) and then use

plot(density(MyData$Column1)) plot(density(MyData$Column2), add=T) 

Or something in this spirit.

Answer 1

use lines for the second one:

plot(density(MyData$Column1)) lines(density(MyData$Column2)) 

make sure the limits of the first plot are suitable, though.

Answer 2

ggplot2 is another graphics package that handles things like the range issue Gavin mentions in a pretty slick way. It also handles auto generating appropriate legends and just generally has a more polished feel in my opinion out of the box with less manual manipulation.

library(ggplot2)  #Sample data dat <- data.frame(dens = c(rnorm(100), rnorm(100, 10, 5))                    , lines = rep(c("a", "b"), each = 100)) #Plot. ggplot(dat, aes(x = dens, fill = lines)) + geom_density(alpha = 0.5)