Hi, here is one easy approach.
1) Right-click in your graph and select Smoother > Save Formula.
2) Look at the formula in the data table... it will have 2 "Spline Eval" pieces, one for each member in the group.... something like this:
3) Copy + paste each individual piece, so that it operates on all rows, not just rows belonging to a specific group, into a new formula column.
***** NOTE: this may involve extrapolation in your case, so heed the relevant interpretive cautions. Each formula will look something like this:
This will produce something like the last 2 columns, which I've named Curve 1 and Curve 2, in your table:
Next, compute the difference in these 2 columns, using a new formula column or a 1-click transform from within Graph Builder, and graph the difference column and the 2 formula columns you previously created vs. x, to view the results:
I've attached a table with illustrative table scripts and formula columns.
Cheers,
Brady