Charting Differences Between Two Lines

Hello:

I have 2 lines of data in a chart, like the past and the current
values. The axis are the years in which the values occur. I have to
draw a line between the two lines, pex: in 2007 the past value was
1000, now, the present value is 1100. In which year of the present
values did the former 1000 value occur? EX:

|
| /
| /
| /___/ This would be the line.
| / /
| /
| /
|__/________

This process will be repeated at least 10x per 27 chats, so, I think I
need some urgent help!

Thanks