%macro grloga(vary=b, a=1, b=.5, lo=-1, hi=1, inc=.1);

  %let &vary = &lo ;
  %do %while (%sysevalf( &&&vary <= &hi, boolean));
    %if (%sysevalf(&&&vary > &lo))  %then goptions gsfmode=append;    ; /* note 2 ; needed */
    %if (%sysevalf(&&&vary >= &hi)) %then goptions gepilog='3B'x;     ; /* note 2 ; needed */

    %if &vary=a %then %grlog(a=&&&vary,b=&b);
    %if &vary=b %then %grlog(a=&a,b=&&&vary);

    %let &vary = %sysevalf(&&&vary + &inc);
  %end;
%mend;