def data_by_artist(dataframe):
    """
    A function that groups and sums statistics by artist.

    :param dataframe: A reference dataframe.
    :return: Result of the grouping.
    """

    dataframe.Artist = dataframe.fillna('NONE').Artist.apply(lambda x: x.split(', '))
    dataframe = dataframe.explode('Artist').groupby('Artist').sum()

    return dataframe