docs: def t_surf_parameter(self, num_frame, xax):
          """
          Surface parameter evolution as a function of time or model.
  
          Parameters
          ----------
          num_frame : integer
              Number of frame to plot this plot into.
          xax : string
              Either model or time to indicate what is to be used on the
              x-axis
  
          """
  
          pyl.figure(num_frame)
  
          if xax == 'time':
              xaxisarray = self.get('star_age')
          elif xax == 'model':
              xaxisarray = self.get('model_number')
          else:
              print('kippenhahn_error: invalid string for x-axis selction. needs to be "time" or "model"')
  
  
          logL    = self.get('log_L')
          logTeff    = self.get('log_Teff')
  
          pyl.plot(xaxisarray,logL,'-k',label='log L')
          pyl.plot(xaxisarray,logTeff,'-k',label='log Teff')
          pyl.ylabel('log L, log Teff')
          pyl.legend(loc=2)
  
  
          if xax == 'time':
              pyl.xlabel('t / yrs')
          elif xax == 'model':
              pyl.xlabel('model number')