def check_day_frequency(df, col_name='ds'):
    """Creates a bar chart showing the frequency of the days of the week.

    Used to check that only business days are included in the dataset, and
    that there is a roughly equal distribution of entries across the week.

    :param df:               A DataFrame containing the time series to check
    :type  df:               pd.DataFrame

    :param col_name:     The name of the column of interest
    :type  col_name:     string
    """
    df['day'] = df[col_name].apply(lambda x: x.weekday_name)
    print(df['day'].value_counts())
    df['day'].value_counts().plot(kind='bar')
