add a secondary y axis to ggplot2 plots - make it perfect

Question

Adding a secondary y axis, scaled one of the original y axis. This topic is not new. It has been touched times, for example on this ggplot2 google groups thread. Following Hadley's advice, I tried to add the secondary y axis by geom_vline, geom_segment, and geom_text. But, it is still ugly.

So I would ask for your help on making it perfect. I think many ggplot2 users would be interested in this topic and prefer any your expertise or contributions. Thanks in advance.

#########################################
# what I have gotten.
library(ggplot2)

# build up a box plot
p <- ggplot(mtcars, aes(factor(cyl), mpg)) 

# add the secondary y axis on right side of the plot
p + geom_boxplot() + geom_vline(xintercept = 3.5) + 
 geom_segment(aes(x=3.49, y=c(7,14,21,28), xend = 3.52, yend = c(7,14,21,28))) +
 geom_text(aes(x=3.55, y=c(7,14,21,28), label=c(7,14,21,28)))

Answer 1

To avoid hacking, you might use facet_grid instead. Depending on your data, you can customize it pretty well, to make it into more general secondary axis.

 library(ggplot2)
 ggplot(mtcars, aes(factor(cyl), mpg)) + 
   geom_boxplot() + 
   facet_grid(cyl ~., scales = "free")