code: def fit(self, range, function=None):
          
          if function is not None:
              self.fit_function = function
          cmd = 'FITT', Integer(min=0, max=100), Integer(min=0, max=100)
          self._write(cmd, start, stop)
docs: Fits a function to the active display's data trace within a
          specified range of the time window.
  
          E.g.::
  
              # Fit's a gaussian to the first 30% of the time window.
              lockin.fit(range=(0, 30), function='gauss')
  
          :param start: The left limit of the time window in percent.
          :param stop: The right limit of the time window in percent.
          :param function: The function used to fit the data, either 'line',
              'exp', 'gauss' or None, the default. The configured fit function is
              left unchanged if function is None.
  
          .. note::
  
              Fitting takes some time. Check the status byte to see when the
              operation is done. A running scan will be paused until the
              fitting is complete.
  
          .. warning::
  
              The SR850 will generate an error if the active display trace is not
              stored when the fit command is executed.