Partly transparent area graph

I want have the area in an area graph to make itself shown, but the gridlines behind should still be visible. There is an option to adjust transparency, but it is dimmed and I don't know how to activate it.
Anyone knows how?


NSV

"Although the Fill Effects dialog box includes Transparency controls, these controls are disabled." - oh well.

Thanks for the tip, now I know the state of things so I don't waste time looking for something impossible.

NSV