Largest value seen by any previous invocation of the MAX function since
the last ERASE procedure call; i.e. the maximum MAX value.  Value is
automatically updated whenever the MAX function or the SCALEY procedure
is called.

Use MINMIN and MAXMAX to set the minimum and maximum values for
autoscaling when a plot involves two or more traces and it cannot be
determined which trace will achieve the lowest/highest value.  For
example,

     SCALEY(expr1)  &  SCALEY(expr2);
     YMIN = MINMIN;    YMAX = MAXMAX;
     PLOT(expr1)  &  PLOT(expr2);
