Try something like this:
dt = open("$SAMPLE_DATA\Big Class.jmp");
biv=dt << Bivariate(
Y( :height ),
X( :weight ),
Fit Spline( 0.1, Standardized, {Line Color( {208, 64, 86} )}, {Save Predicteds} ),
By( :age )
);
To format the column you can send the Format message to the new column, which will be the last column in the table.
column(dt, ncol(dt))<< format("d/m/y");
-Jeff
-Jeff