While preparing a class exercise involving the use of overlaying of histogram, I searched Google on possible article or discussion on the said topic. Luckily, I found a blog where the author demonstrated an R function to create an overlapping histogram. However, a comment from a guy also showed the same output using transparency. Below were the sample codes that can be used to generate overlapping histogram in R as based on the blog and the viewers comment.
# Histogram Grey Color
hist(h1, col=rgb(0.1,0.1,0.1,0.5),xlim=c(0,10), ylim=c(0,200), main="Overlapping Histogram")
hist(h2, col=rgb(0.8,0.8,0.8,0.5), add=T)
# Histogram Colored (blue and red)
hist(h1, col=rgb(1,0,0,0.5),xlim=c(0,10), ylim=c(0,200), main="Overlapping Histogram", xlab="Variable")
hist(h2, col=rgb(0,0,1,0.5), add=T)