Zrange

The set zrange command sets the vertical range that will be displayed. This command turns z axis autoscaling off. The zrange is used only by splot and is ignored by plot.

This range may also be specified on the splot command line.

Syntax: 

        set zrange [{<zmin> : <zmax>}]

where <zmin> and <zmax> terms are constants or expressions.

Both the <zmin> and <zmax> terms are optional. Anything omitted will not be changed, so 

        set zrange [2:]
changes zmin to 2 without affecting zmax.