Histograms and Density Plots in ggplot2

Now you will learn the ggplot2 syntax for building and customizing histograms.

The package plyr is used to calculate the average weight of each group :

library(plyr)
mu <- ddply(df, "sex", summarise, grp.mean=mean(weight))
head(mu)

##   sex grp.mean
## 1   F    54.70
## 2   M    65.36