excel area graph - make translucent

I am trying to make an area chart in Excel that shows the overlap between the two data series in a third color, a sort of translucent color so you can see where the otherwise hidden series is (like tracing paper).

This is what my graph looks like


I know I can just say "None" so the one series won't have a fill and you will just see the line going through, but I really want it to look translucent, so the color under and the line, or at least the line, shines through the top color.

Any ideas??