problem with horizontal lines in chart

Excel 2003
I have a standard clustered column chart and it shows numbers (in time)
gathered for several different groups each month (so group A has bars for Jan
- June [eventually - Dec] then group B, etc)
I have put a horozontal line across denoting the target number for all of
them (same each month so it's one line across)
I would like to add another horozontal line for EACH group showing it's YTD
average. So this line wouldn't go across the entire chart, only across the
section for that particular group, but I don't know how to do that. I figured
it would be something to do with where the line starts and stops on the x
axis?? How can I do that? Or is it do-able?
thanks, Meenie