I'm not sure if you could bring circles onto the chart...i was thinking about it for a bit,but the only way i can think of is calcuating the position and adding an autoshape, but that could be done using the values given in the chart?, a bit of trial and error would be required tho i would of thought.

The only problem with that idea i believe, is ... well i've noticed in most programs the shapes like to go behind other things, if excel lets you put the autoshapes above the chart then it could work...maybe!

i just had quick link on those sites, and the first one states it all done using autoshapes...