Charting Running Totals

[SBpsc]

Hi:

I am using CR 10 Pro. I have two running total values (Planned and Actual).
I created a formula called Variance = (#Planned - #Actual)

I need to chart the Variance against Time.

But, the Variance field is not showing under the available fields for charting. Can someone tell me how and if this can be done?

Thanks,
SB