Update: I found a solution on http://www.mac-forums.com/forums/os-x-apps-games/242997-plots-octave-dont-work.html.
It's simply to put this:
## commands executed on startup
into an ~/.octaverc file. It works for octave, but not for QtOctave, but that's acceptable.
Update 2: It even works for qtMatlab, now I only need to get the inferior octave thingy in AquaEmacs to work and I'm all set to work in perfect comfort :-)