How to add % difference in dual trend lines

Hey Guys
I am preparing two trend lines in one chart. For ex Say $ (Y axis) for client A in Q1, Q2, Q3 and Q4 (X axis) as one line and same metrics for Client B.

Is there a way to automatically show the % difference between the two clients for a given X axis value

So ideally we should have two lines and a % line connecting A and B showing how much Client A is higher than Client B in (say) Q1, Q2 etc

Thanks much
Cheers

Is this the sort of thing you mean?
http://www.andypope.info/charts/Hilolabel.htm

Also when you say 'trend lines' do you mean period on period or a equational trend line?