Why isn't this what you need, in getting the max values for each axis?
Names Default To Here( 1 );
dt = Open( "$SAMPLE_DATA/big class.jmp" );
gb = dt << Graph Builder(
Variables( X( :weight ), Y( :height ) ),
Elements( Points( X, Y, Legend( 6 ) ) )
);
show(report(gb)[AxisBox(1)] << get max,
report(gb)[AxisBox(2)] << get max
);
Jim