Yrange

The set yrange command sets the vertical range that will be displayed. This command turns y axis autoscaling off.

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

Syntax: 

        set yrange [{<ymin> : <ymax>}]

where <ymin> and <ymax> terms are constants or expressions.

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

        set yrange [:10]
changes ymax to 10 without affecting ymin.