[R] boxplot via plot command

[Antje]

Hi folks,

I've just discovered that the following code leads to boxplot (surprisingly to me).
Can anybody explain to me why? Is this documented somewhere? I've never 
consider this option before.

x <- rnorm(300)
l <- c(rep("label1",100), rep("label2",50), rep("label3",150))

df <- data.frame(as.factor(l), x)
plot(df)


Thank you!
Antje