Line Graph: Differences between series

I have two sets of numbers that I've graphed in a basic line graph: Series A & Series B. I want to edit the line graph and put a data table at the bottom that automatically caluclates and displays the value of A minus B.

The attached file shows what I want the graph to look like, but I've manually inserted text boxes below the X Axis to display the differences; I'd like to have them automatically calculated as I enter new values for Series A & B.

Does anyone have advice on this?

Thanks,
Mike