Since OBI renders things dynamically the linebar combo is about as close as you can get. The scales would be the only thing which would allow a distance between the the sum of A+B....because OBI obviously places the sum otherwise at the precise A+B point. Because that's its value!

This isn't Powerpoint where you push around things based on pixels.

If your client wants something non-standard and insists on it then write it in D3 or one of the other charting engines.

tl;dr - this is "requirement" isn't a requirement but a wish based on doing whatever they want in Excel/Powerpoint

Ok imagine this:

A = 10, B = 20.

You put them stacked. That makes...30. So B ends and 30 in terms of rendering.

C = A + B = 10 + 20 = 30.

You put that on the line side. So C gets rendered at 30. Which means on yhe level where B ends on the stacked side.

30 = 30 so it overlaps. Logical I would say, no?

So for C not to overlap you would have to change the scaling for the line side of the chart so that the number appears more detached.

That said this is just visual cheating  and has nothing to do with analytical data rendering.

Write it in D3 or do it in Excel.