You can use Fit Y by X and then set the Group By, and then do a Flexable==>Fit Spline. Each group will have it's own Lambda slider
Names Default To Here( 1 );
dt = Open( "$SAMPLE_DATA/big class.jmp" );
dt << Bivariate(
Y( :height ),
X( :weight ),
Fit Where( :sex == "F", Fit Spline( 1000, {Line Color( {212, 73, 88} )} ) ),
Fit Where( :sex == "M", Fit Spline( 1000, {Line Color( {66, 112, 221} )} ) )
);
Jim