[math-fun] Drawing pictures with an iMac

I have a nice iMac but other than the terminally buggy and limited app "Grapher" I don't have a way to draw arbitrary pictures on it. I used to have GL (what gave rise to openGL) under Unix, and could just write a program to do pretty much anything I wanted. (The key is having the command "Paint pixel (K,L) the color (R,G,B).") Can someone please tell me how to do this on my iMac ? ( What I've read about computer graphics on my iMac in Apple documentation sounds w-a-a-a-a-a-a-y more complicated than it used to be, with all manner of platforms upon platforms. Is that really necessary? After all, there's a form of Linux on the iMac and I'd be happy to program with that. ) Thanks, Dan