barplot with some 0 frequencies

Hi all,

I am back with a new question !
I recorded the occurence of 4 differents event on 20 places for a given time
period.
Now, I want to do some barplot of the frequency of theses events for each
place, so it should be easy. My problem is that I want to see the
frequencies of the 4 events on my barplots even if the frequency of some of
them is 0. 
How could I do that ?