How to plot two histograms together in R

Question

I have two data frames, one each for cucumbers and carrots, and I'm using R. Each data frame has a single numerical column that records the length of each measured cucumber and carrot (100k total) (total: 50k cucumbers).

I want to plot the lengths of the cucumbers and the carrots on the same histogram. I suppose I also need some transparency because they overlap. Additionally, because there are varying numbers of cases in each category, I must use relative frequencies rather than absolute numbers.