Annotations - R Graphics - page 201

Here we'll take the average height for males and females and store it in a data frame, hw_means .
Then we'll draw a horizontal line for each, and set the linetype and size ( Figure 7-8 ):
library(plyr) # For the ddply() function
hw_means <- ddply(heightweight, "sex" , summarise, heightIn = mean(heightIn))
hw_means
sex heightIn
f 60.52613
m 62.06000
p + geom_hline(aes(yintercept = heightIn, colour = sex), data = hw_means,
linetype = "dashed" , size = 1 )
Figure 7-8. Multiple lines, drawn at the mean of each group
If one of the axes is discrete rather than continuous, you can't specify the intercepts as just a
character string—they must still be specified as numbers. If the axis represents a factor, the first