How to make a Running Total function, Jaspersoft Server

[a.schubert]

How do you create a running total function in jaspersoft server. I am in the ad hoc editor and have already created the Percent of column, but have not figured out how to create the function to create the running total. 

an example would be 

customer	sales	running total
joes cake	3000	54%
twinkie	2000	90%
ma and pa's cookies	500	100%
total	5500	100%

any help is greatly appreciated