def _fill_histogram(idf, hist, features):
    """Fill input histogram with column(s) of input dataframe.

    Separate function call for parallellization.

    :param idf: input data frame used for filling histogram
    :param hist: empty histogrammar histogram about to be filled
    :param list features: histogram column(s)
    """
    name = ":".join(features)
    clm = features[0] if len(features) == 1 else features
    # do the actual filling
    hist.fill.numpy(idf[clm])
    return name, hist