Map density to fill not colour

[alanocallaghan]

Would be very cool to be able to set pch=21 and then map the point fill to density. Perhaps this is obvious but not to me. I tried

ggplot(mtcars) + geom_pointdensity(shape = 21, aes(mpg, cyl, fill=stat(density)))

But I get

Error: Aesthetics must be valid computed stats. Problematic aesthetic(s): fill = stat(density). 
Did you map your stat in the wrong layer?