The reason is that I'm only clearing the few pixels around the crosshair before redrawing. It seems like you would get that problem if your browser / computer were lagging behind. What browser are you using?
Firefox/Chrome both do it for me, on the three different computers I use. The charts/orders seem to update in real time, and without lag. Just that cross hair. I wasn't sure if it was a plotting tool or had some other use.