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')
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