[R] overlay two histograms ggplot

[Elahe chalabi]

Hi all,

How can I overlay these two histograms? 

ggplot(gg, aes(gg$Alz, fill = gg$veg)) + geom_histogram(alpha = 0.2)
ggplot(tt, aes(tt$Cont, fill = tt$veg)) + geom_histogram(alpha = 0.2)

thanks for any help!
Elahe