Overlapped Histograms

alpha = 0.5
col = rgb(0,1,0,alpha)
hist(rnorm(1000),nclass=50,col=col,xlim=c(-3,6))

alpha = 0.5
col = rgb(1,0,0,alpha)
hist(3+rnorm(1000),nclass=50,col=col,add=T)