How to set fill alpha in geom_box to number of observations in coefficient

I need to draw a set of graphs where data is grouped by factors. I would also like to set a value alpha

for each block to represent the number of observations in each factor. The more observations for a given factor, the higher alpha

.

Using the data mpg

as an example ...

Drawing a plot is simple:

library(ggplot2)
ggplot(mpg, aes(x=factor(class), y=displ)) + 
   geom_boxplot(aes(fill="red", alpha=10)) +
   scale_fill_manual(values=c("red"="red"),guide="none") +
   scale_alpha(range=c(0,1), guide="none")

      

        
        
        
      

    

And calculating the number of observations for each is class

simple using plyr

:

ddply(mpg, .(class), nrow)
       class V1
1    2seater  5
2    compact 47
3    midsize 41
4    minivan 11
5     pickup 33
6 subcompact 35
7        suv 62

      

        
        
        
      

    

I'm sure ggplot2

there is a clever way to do this inside , but I've run out of ideas.

Thank!