add vertical line of the mean to density plot

Question

Following the example from this website, we can generate a density plot for several factors within a data.frame

library(sm)
attach(mtcars)
sm.density.compare(mpg, cyl, xlab="Miles Per Gallon")

My question is very simple, how can be add a vertical line for each factor which represents the median or the mean?

Answer 1

This is an example for the mean. To calculate the median, simply replace "FUN = mean" with "FUN = median" in the aggregate function.

library(sm)
attach(mtcars)
sm.density.compare(mpg, cyl, xlab="Miles Per Gallon")

means <- aggregate(mpg ~ cyl, FUN = mean)
abline(v = means[1,2], col = 2)
abline(v = means[2,2], col = 3, lty = 2)
abline(v = means[3,2], col = 4, lty = 3)