def combine_df(df1, df2):
    """ Join the data frames together.

    :param df1:   pandas data frame 1.
    :param df2:   pandas data frame 2.

    :return:    a single pandas data frame.
    """

    # Combine the two data frames with one another.
    df1["j"] = 1
    df2["j"] = 1
    out = df1.merge(df2)
    out = out.drop(columns="j")

    return out