def plot(self,tt=0):
        """
        NAME:
           plot
        PURPOSE:
           plot the velocity distribution
        INPUT:
           t= optional time index
        OUTPUT:
           plot of velocity distribution to output device
        HISTORY:
           2011-06-27 - Written - Bovy (NYU)
        """
        xrange= [self.vRgrid[0],self.vRgrid[len(self.vRgrid)-1]]
        yrange= [self.vTgrid[0],self.vTgrid[len(self.vTgrid)-1]]
        if len(self.df.shape) == 3:
            plotthis= self.df[:,:,tt]
        else:
            plotthis= self.df
        bovy_plot.bovy_dens2d(plotthis.T,cmap='gist_yarg',origin='lower',
                              aspect=(xrange[1]-xrange[0])/\
                                  (yrange[1]-yrange[0]),
                              extent=[xrange[0],xrange[1],
                                      yrange[0],yrange[1]],
                              xlabel=r'$v_R / v_0$',
                              ylabel=r'$v_T / v_0$')
NAME:
           plot
        PURPOSE:
           plot the velocity distribution
        INPUT:
           t= optional time index
        OUTPUT:
           plot of velocity distribution to output device
        HISTORY:
           2011-06-27 - Written - Bovy (NYU)