XMAX = expr;            Set maximum value for X axis scaling

The XMAX control specifies the maximum X axis value for scaling
subsequent plots.  This parameter is ignored unless an X axis file
has been specified (see XAXIS control).  As with the YMIN and
YMAX controls, autoscaling on the X axis can be invoked by
entering either:

     SCALEX(xaxisvar);
  or
     XMIN = MIN(xaxisvar) & XMAX = MAX(xaxisvar);

where 'xaxisvar' is the name of the variable being used for the X axis.

The minimum and maximum values for the X axis must be set before the
PLOT procedure is invoked.
