r - How to increase size of the points in ggplot2, similar to cex in base plots?

Question

I have the following dummy code:

dt<-data.frame(country=letters[1:20],val=rnorm(20),siz=rnorm(20))
qplot(x=country,y=val,data=dt,geom="point",size=siz)

Now I want to increase the relative size of the points, since the resulting smallest point is too small. Is this possible to do by changing one parameter, like cex in base plots?

Answer 1

You want scale_size() and it's argument range (or to according to the ggplot website):

qplot(x=country,y=val,data=dt,geom="point", size=siz) + 
    scale_size(range = c(2, 10))

Fiddle with the range to get suitable minimum/maximum sizes.