code: def plot(self,tt=0):
          
          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'$v_T / v_0$')
docs: 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)