Hi @CircularPuppy35 ,
I think the problem might start with the fact that your report(biv) variable has multiple elements to it. If you look through the code below where I use the Big Class.jmp file and use :sex and :age as BY variables, you can see that the variable rbiv has 12 elements in it, and you need to access the ith element first, with something like rbiv[i]. See the code below. And, if you just loop through however many elements there are in rbiv, it'll change the Y-axis to all be Log.
Names Default To Here( 1 );
dt = Open( "$SAMPLE_DATA/Big Class.jmp" );
biv = Bivariate( Y( :height ), X( :weight ), By( :sex, :age ), Fit Special( yTran( "Log" ) ) );
rbiv = biv << report;
For( i = 1, i <= N Items( rbiv ), i++,
rbiv[i][Outline Box( 1 )][Axis Box( 1 )] << {Scale( "Log" )}
);
Hope this helps!,
DS