I think one fairly easy option which should work is to create new column with ",\!N" as separator between different values.
Names Default To Here(1);
dt = Open("$SAMPLE_DATA/Big Class.jmp");
:name << Label(0);
new_col = dt << New Column("LABEL", Character, Nominal, Formula(
Char(:height) ||",\!N" || Char(:weight)
));
new_col << Label(1);
gb = dt << Graph Builder(
Size(1029, 499),
Variables(X(:name), Y(:height)),
Elements(Bar(X, Y, Legend(6), Label("Label by Row")))
);
-Jarmo