code: def add_frequencyseries(self, spectrum, dB=True, power=False, **kwargs):
          
          # parse spectrum arguments
          kwargs.setdefault('label', spectrum.name)
          # get magnitude
          mag = numpy.absolute(spectrum.value)
          if dB:
              mag = to_db(mag)
              if not power:
                  mag *= 2.
          # get phase
          phase = numpy.angle(spectrum.value, deg=True)
          # plot
          w = spectrum.frequencies.value
          mline = self.maxes.plot(w, mag, **kwargs)[0]
          pline = self.paxes.plot(w, phase, **kwargs)[0]
          return mline, pline
docs: Plot the magnitude and phase of a complex-valued `FrequencySeries`
  
          Parameters
          ----------
          spectrum : `~gwpy.frequencyseries.FrequencySeries`
              the (complex-valued) `FrequencySeries` to display
  
          db : `bool`, optional, default: `True`
              if `True`, display magnitude in decibels, otherwise display
              amplitude.
  
          power : `bool`, optional, default: `False`
              give `True` to incidate that ``spectrum`` holds power values,
              so ``dB = 10 * log(abs(spectrum))``, otherwise
              ``db = 20 * log(abs(spectrum))``. This argument is ignored if
              ``db=False``.
  
          **kwargs
              any other keyword arguments accepted by
              :meth:`~matplotlib.axes.Axes.plot`
  
          Returns
          -------
          mag, phase : `tuple` of `lines <matplotlib.lines.Line2D>`
              the lines drawn for the magnitude and phase of the filter.