def make_comparison_dataframe(historical, forecast):
    """Join the history with the forecast.
    
       The resulting dataset will contain columns 'yhat', 'yhat_lower', 'yhat_upper' and 'y'.
    """
    return forecast.set_index('ds')[['yhat', 'yhat_lower', 'yhat_upper']].join(historical.set_index('ds'))