2 different types of data sets displayed on 1 graph

I have a district that I wish to show performance (sales)in the months from
July to November and comapred this with 2004 and 2005. Now on top of this I
want to display on the same graph a second variable say number of customers
for each month and each year, in order to bring some greater relevance to the
static bar graphs that I propose doing for the first part.

So I have been assisted on this discussion group to have a bar graph that is
set up with the months as rows and the 2004 and 2005 as columns for the sales
data BUT how do I now display the second variable 'number of customers' as
say a line graph over the 2004 and 2005 bar graph sets?
Mike

Can I then add 11 more districts to the same graph or is that becoming too
messy / congested as a graph?
Mike