I am not sure how to create the matrix without creating the column in the data table, but here is a method that functionally will do what you want
Names Default To Here( 1 );
dt = Open( "$SAMPLE_DATA/Big Class.jmp" );
// Get the groupings for the sex column
Summarize( dt, bysex = by( :sex ) );
obj = Bivariate( Y( :Weight ), X( :Height ) ) << Group by( :sex );
obj << Fit spline( 0.1, Standardized, {Save Predicteds} );
// Create the matricies for the groups and delete the
// columns from the data table
For( i = N Items( bysex ), i >= 1, i--,
Eval(
Parse(
bysex[i] ||
"_Predicted_Matrix = column(dt,N Col(dt))<<get values;"
)
);
dt << delete columns( N Cols( dt ) );
);
Show( f_Predicted_Matrix, m_Predicted_Matrix );
Jim