If the vertical scale is changed when the Expand 
function is set to ground, the signal will expand 
from the ground level. The ground level position 
does not change when the vertical scale is 
changed.  
If the vertical scale is changed when the Expand 
function is set to center, the signal will expand 
from the center of the waveform window. The 
ground level position will suit to match the signal 
position.