def define_series(series, source, indexes):
    """If series is empty returns source_columns excluding the column
    where column == index. Otherwise returns the series.split(',')

    Args:
        series (str): string that contains the names of the
            series to keep from source, separated by `,`
        source (DataFrame): pandas DataFrame with the data series to be
            plotted
        indexes (lst(str)): name of the series of source to be used as index.

    Returns:
        list of the names (as str) of the series except index
    """
    if not series:
        return [c for c in source.columns if c not in indexes]
    else:
        return series.split(',')