code: def plot(self, threshold=None, **kwargs):
          
          delays, responses = self.delay_response_series
          pyplot.plot(delays, responses, **kwargs)
          pyplot.xlabel('time')
          pyplot.ylabel('response')
          if threshold is not None:
              threshold = numpy.clip(threshold, 0., 1.)
              cumsum = numpy.cumsum(responses)
              idx = numpy.where(cumsum >= threshold*cumsum[-1])[0][0]
              pyplot.xlim(0., delays[idx])
docs: Plot the instanteneous unit hydrograph.
  
          The optional argument allows for defining a threshold of the cumulative
          sum uf the hydrograph, used to adjust the largest value of the x-axis.
          It must be a value between zero and one.