def _get_point_sizes_for_dpair_scatter(df):
    """
    A helper method that scales scatterpoint sizes corresponding to TOI column. The largest point gets a size of 200;
    others get smaller linearly.

    :param df: dataframe with TOI column

    :return: df with an extra column Size that can be used in matplotlib as the kwarg 's'
    """

    largest = df.TOI.max()
    df.loc[:, 'Size'] = df.TOI / largest * 200
    return df