Include average information with each chart. This could include mean, median, and mode.
Hey. I had a similar need : I wanted to display the average curve of a serie, and the series itself, on the same chart.
This is what I did : http://jsfiddle.net/6gXfu/ I used "groupPixelWidth" and have a pretty nice result. You can change groupPixelWidth and the zoom to see how it does.
I hope this has been of some help !
(I even use it in a live chart -> updating from a server every 'n' seconds; and it works fine)