YMAX = expr;        Set maximum Y axis value for scaling

The YMAX control specifies the maximum Y axis value used for
scaling plots.  Autoscaling for a plot of expression 'plotexpr' can
be accomplished by specifying:

     YMIN = MIN(plotexpr) & YMAX = MAX(plotexpr);
  or
     SCALEY(plotexpr);

The minimum and maximum values for the Y axis must be set
before an expression is plotted and the statements settings these
values must not be grouped with the statement invoking the PLOT
procedure.
