Clustered Cylinder Graph

I have created a Clustered Cylinder graph which is very simple using the following data.

Backlog 21/12/10 22/12/10 23/12/10 24/12/10 25/12/10 26/12/10 27/12/10

1071 512 295 20 0 0 0 0


I would like to add an additional column displaying the average and display this value as a line on the graph - is this possible?