title: How to named slice of a two-dimensional array in python
body: <pre class="lang-py prettyprint-override"><code>points = [ (1, 2), (3, 4), (5, 6), (7, 8) ]
  points = np.array(points)
  plt.plot(points[:,0],points[:,1],'ro')
  </code></pre>
  
  <p>how to named this slice（[:,0] and [:,1]） ?</p>