multi-class histogram?

I am new to R, and I couldn't find the answers to my question in a faq. 
This could however be because I didn't know what to look for...:)

I have three classes of data, data for bacteria, archaea and eukaryotes. 
I wish to display these in a histogram where all of the values are used 
to calculate each column. But, I want each column split in three, where 
the size of each coloured area represents the proportions of the values 
in that column that comes from each of the three classes. Do you have 
any tips on how I do this? 

Karin

Karin Lagesen, PhD student
karin.lagesen at medisin.uio.no
http://www.cmbn.no/rognes/