Tablix with Cumulative Summing

[cs2009]

Using SSRS 2008 R2.

I have data returned from a query like this:

Month Order Count
Jan 5
Feb 10
Mar 15
Apr 20

I need to sum the order counts over time (a cumulative count) like this:

Jan Feb Mar Apr
5 15 30 50

Is this possible to do with the tablix control? If so, please explain how.

 

[cs2009]

The RunningValue() function is what I need.