Forecast all time series in the group. See the
        `TimeSeries.forecast()` method for more information.

def forecast(self, horizon, **kwargs):
        '''Forecast all time series in the group. See the
        `TimeSeries.forecast()` method for more information.'''
        return DataFrame({ name: series.forecast(horizon, **kwargs) \
            for name, series in self.groups.iteritems() })