Wow, believe it or not, such a nice site survey tool doesn't even exist on standard Linux (GUI and everything). Even on Windows there is not much choice, best of them being inSSIDer. I think you should get some ideas from their UI (AP list, signal/time graph...). You already have enough data, just add it to GUI. And make the graphs as big as possible please, you should make a better use of screen real estate. And I know this is alpha/beta stage, I'm just suggesting improvements.

Video: http://vimeo.com/3284530