same data as expo_avgs, but with keys as the average age
        of the data -- assuming evenly spaced data points -- rather
        than decay rates

def lag_avgs(self):
        '''
        same data as expo_avgs, but with keys as the average age
        of the data -- assuming evenly spaced data points -- rather
        than decay rates
        '''
        if not self.interval:
            return
        interval = self.interval.mean
        return dict([(interval/alpha, val) 
            for alpha, val in self.get_expo_avgs().items()])