Box plot showing mean as a line

Question

Is it possible to create a boxplot that shows both mean and median as a line with the standard boxplot function of R ? My current solution displays the mean as a cross:

set.seed(1234)
values <- runif(10,0,1)
boxplot(values)
points(mean(values),col="red",pch=4,lwd = 4)

Answer 1

The default plotting of boxplot makes the width of the box go from 0.8 to 1.2 in the x-axis.

You can hence draw a line for the mean with the below code:

lines(c(0.8, 1.2), rep(mean(values), 2), col="red", lwd = 2)

Answer 2

For the sake of completeness, you could also overplot:

set.seed(753)
df <- data.frame(y=rt(100, 4), x=gl(5, 20))
bx.p <- boxplot(y~x, df)
bx.p$stats[3, ] <- unclass(with(df, by(y, x, FUN = mean)))
bxp(bx.p, add=T, boxfill="transparent", medcol="red", axes=F, outpch = NA, outlty="blank", boxlty="blank", whisklty="blank", staplelty="blank")

Explanation via @scs:

bxp$stats returns a matrix that contains the lower whisker, the lower hinge, the median, the upper hinge and the extreme of the upper whisker for each boxplot. The solution above overwrites the median specified in bx.p$stats[3, ] with the mean value. The bxp function is a function to plot boxplot objects.

Result: