How can I calculate the cumulative difference between side-by-side bars in a bar chart

[Niki Nichols 2]

I am taking large amounts of raw daily gas volume data and aggregating it to compare against my forecasted volume data. I have made a side-by-side bar chart comparing my actuals vs. my forecast for any given month. I want to make this a combination chart that has a cumulative year-to-date variance calculation that showsTotal Actual Volume - Total Forecasted Volume.

Can I do this with a custom expression I've attached a photo below of the data I have. In January, my custom expression should = 1,272,986 - 1,268,523 = 4,463. In February, my customexpression should = 4,463 + (1,290,170 - 2,042,977) = 4,463 + (-752,807) = -748,344. Meaning I am 748,344 units below my forecasted volume so far for the year.

 

Help! Thank you in advance!