docs: def t_lumi(self,num_frame,xax):
          """
          Luminosity 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"')
  
  
          logLH   = self.get('log_LH')
          logLHe  = self.get('log_LHe')
  
          pyl.plot(xaxisarray,logLH,label='L_(H)')
          pyl.plot(xaxisarray,logLHe,label='L(He)')
          pyl.ylabel('log L')
          pyl.legend(loc=2)
  
  
          if xax == 'time':
              pyl.xlabel('t / yrs')
          elif xax == 'model':
              pyl.xlabel('model number')