code: def plot(self,plane,axis=None,**kargs):
          
          if(axis == None):
              axis = plt.gca()
          if(plane == 'xy'):
              axis.plot(self._pos[:,0], self._pos[:,0], 'k.', **kargs)
          elif(plane == 'xz'):
              axis.plot(self._pos[:,1], self._pos[:,2], 'k.', **kargs)
          elif(plane == 'yz'):
              axis.plot(self._pos[:,2], self._pos[:,2], 'k.', **kargs)
docs: Use this method to plot the set of particles stored by the Particles class.
          In order to plot the distribution of Particles, a *plane* parameter must be given.
          "plane" is one of the available orthogonal projections of the particles:  
          |'xy'|'xz'|'yz'|. If there is multiple axes defined, the active one can be 
          selected using the axis parameter. If axis paremeter is None (default), the 
          distribution of particles is plotted in the active axis, which is returned by 
          the matplotlib.pyplot.gca() method.