def METHOD_NAME():
    g1 = Graph('1', {})
    g2 = Graph('2', {})
    g3 = Graph('3', {})
    for i in range(5):
        if i >= 3:
            g1.add_data_point(i, None)
        g2.add_data_point(i, i)
        if i <= 2:
            g3.add_data_point(i, [-i, -2 * i])
    x, ys = _combine_graph_data([g1, g2, g3])
    assert x == [0, 1, 2, 3, 4]
    assert ys == [[None, None, None, None, None],
                  [0, 1, 2, 3, 4],
                  [0, -1, -2, None, None],
                  [0, -2, -4, None, None]]