How to overlay density plots in R?

To r overlay plots, you can do the following:

In base R graphics, you can use the lines() function. But make sure the limits of the first plot are suitable to plot the second one.

For example:

plot(density(mtcars$drat))
lines(density(mtcars$wt))
Output:
In ggplot2, you can do the following:
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)