title: grid zorder seems not to take effect (matplotlib)
body: <p>I am trying to to plot scatter points on a grid using python's matplotlib (<code>1.4.3</code> but tried on other versions without success too). Everything works except the grid <code>zorder</code> position. I set grid's <code>zorder</code> to be lower that that of <code>scatter</code> but the grid still covers the scatter points.</p>
  
  <p>Here is a simple code to reproduce the problem:</p>
  
  <pre><code>import numpy as np
  from matplotlib import pyplot as plt
  
  # create points
  pnts = np.array([[1., 1.], [1., 1.2]])
  
  # create figure
  plt.grid(True, linestyle='-', color='r', zorder=1)
  plt.scatter(pnts[:,0], pnts[:,1], zorder=2)
  plt.ylim([0, 2])
  plt.xlim([0, 2])
  plt.savefig('testfig.png', dpi=350)
  </code></pre>
  
  <p>I get this:
  <img src="https://i.stack.imgur.com/qZzSC.png" alt=""><br>
  If you zoom in on the dots, you can see the grid is on top, although it has lower <code>zorder</code>:
  <img src="https://i.stack.imgur.com/cBrrv.png" alt="enter image description here">  </p>
  
  <p>Is this a bug or am I doing something wrong?</p>