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 (as a fraction of the z-range) above the xy-plane. The default value is 0.5. Negative values are permitted, but tic labels on the three axes may overlap.
To place the xy-plane at a position 'pos' on the z-axis, ticslevel should be set equal to (pos - zmin) / (zmin - zmax).
Syntax:
set ticslevel {<level>} show tics
See also set view.