How to GIMP it!

Hi guys, I’m having a little issue with working the GIMP image manipulator programme. I saw back there in the correlation thread someone put together a picture where two separate price charts are displayed as an overlay on the same frame: like two layers, only with the front somewhat transparent and the back chart(s) visible? Have tried mucking about all those buttons still cant figure which it is, just one button …

Will try to PM the wizz concerned, would really appreciate it in the meantime if someone gives it a go? Thanx.