[R] Transparent colors OR two series on one histogram

[Nathaniel Street]

Hi,

I want to be able to plot a single histogram of a measured trait with 
trait values from two conditions on the same histogram to allow easy 
comparison.

I have previously done this in excel by plotting the two series on a 
single bargraph having calculated frequencies in bins. You then get one 
condition plotted immediately to the right of the other. I hope that 
makes sense?

I don't know if this is possible in R?

If not, I could plot the second one on top of the first i.e.

hist(x)
hist(y,add=TRUE)

but i would need to set one as having a semi-transparent color so that 
where the two sets of are plotted on top of each other, you can see the 
frequency of the first series 'behind' the second.

Can anyone help me do this? It would be another step towards never using 
excel again :)

Thanks