set.seed(1)
h1<-hist(rnorm(1000,100,5))
h2<-hist(rnorm(1000,99,5))
plot(h2,col=rgb(255,0,0,50,maxColorValue =255),border = NA)
plot(h1,col=rgb(225,225,0,50,maxColorValue =255),add=T,border = NA)