Excel 2007 : Chart a BE point (vertical line)?

So I've searched high and low and cannot get a straight answer on this so Im hoping that someone on this forum can help.

Here is the simple version of the format: X axis is units, Y axis is $. There are 2 lines, revenue and cost. Where they intersect is the break even point. Is there any way to have a vertical line cross this intersection? Ex. BE is 642 units, how do I plot a vertical line at 642 (x-axis) from 0 to infinity (Y-axis)?

I am so frustrated that it is this difficult to get excel to plot a BE line, its such an obvious thing that all businesses need to evaluate frequently. (and yes, I know how to draw a line, but I want excel to do it as the data can change)