How to plot two histograms together in R?

Answer

carrots <- data.frame(length = rnorm(100000,6,2))
cukes <- data.frame(length = rnorm(50000,7,2.5))#Now, combine your two dataframes into one.  First make a new column in each.
carrots$veg <-'carrot'
cukes$veg <-'cuke'#and combine into your new data frame vegLengths
vegLengths <- rbind(carrots, cukes)#now make your lovely plot
ggplot(vegLengths, aes(length, fill = veg))+ geom_density(alpha =0.2)

Now, if you really did want histograms the following will work. Note that you must change position from the default "stack" argument. You might miss that if you don't really have an idea of what your data should look like. A higher alpha looks better there. Also note that I made it density histograms. It's easy to remove the y = ..density.. to get it back to counts.

ggplot(vegLengths, aes(length, fill = veg))+ geom_histogram(alpha =0.5, aes(y = ..density..), position ='identity')