I see some unpleasant jumping of the zoom point sometimes. Try this:
Ideally the mouse position would be a fixed point of the zooming action, so the ruler should remain motionless at the mouse point. This sorta happens, but the ruler jumps around a bit.
The culprit is the changing width of the y-axis tic labels. Issuing 'set ytics format "%.2f"' largely fixes the jumping. I'm not sure what the best way to fix this is, but I REALLY feel like zooming into the mouse-centered ruler should be stable.