Using splot, one can adjust the relative height of the vertical (Z) axis
using set ticslevel. The numeric argument provided specifies the location
of the bottom of the scale. A zero will put it on the bottom grid and any
positive number somewhere along the z axis.
Syntax:
set ticslevel {<level>}
show tics
where <level> is a non-negative numeric argument. For example,
set ticslevel 0.5
sets the tics level to the default value.
See also set view.