Need help with showing Totals

I have a clustered cylinder chart. Along the x-axis, I have the average test
score per exam per class. For example, in the first cluster I have the
average test score for five exams in the Science class. In the second
cluster, I have the average test score for five exams in the Mathematics
class, etc.

For each average test score, I would like to display the total number of
students that took the test above each score.

In looking through several of the previous answers in this discussion group,
I think I need to add a dummy series and plot the total number of students
against a second y-axis, but I am not sure.

Can someone please show me how this can be done? I am using Excel 2007.

I would greatly appreciate any help.

Thanks,
Bob