The gravity part I understand no problem (physics teacher), the trick is the OVER 1 but how do you not leave a trail on the intro screen I don't see in the code below where you erase the trail.
It's my PLOT eo,fo in line 68 which is at the same co-ordinates as the PLOT e,f was a couple of lines earlier. Because of OVER 1 it's removing that pixel (OVER 1 meaning plotting is toggling the state of the pixel when it PLOTs).
