title: Odd line artefacts appearing in matplotlib mplot3d plot using PolyCollection
body: <p>I am using the mplot3d example which uses PolyCollection for stacking XY-plots, <a href="http://matplotlib.org/examples/mplot3d/polys3d_demo.html" rel="nofollow noreferrer">http://matplotlib.org/examples/mplot3d/polys3d_demo.html</a></p>
  
  <p>However, I am seeing some strange line artefacts in the plots. </p>
  
  <ul>
  <li>How can I remove the horizontal lines which disappear out of the visible area?</li>
  <li>Is there a better way for stacking XY plot along a depth direction?</li>
  </ul>
  
  <p>The following script produces this plot,</p>
  
  <p><img src="https://i.stack.imgur.com/WO2WH.png" alt="Odd lines in mplot3d plot"></p>
  
  <pre><code>from mpl_toolkits.mplot3d import Axes3D
  from matplotlib.collections import PolyCollection
  from matplotlib.colors import colorConverter
  import matplotlib.pyplot as plt
  import numpy as np
  
  zs = []
  fig = plt.figure()
  ax = fig.gca(projection='3d')
  verts = []
  
  # XY data (i.e. "normal line plots")
  count = 4
  for i in range(count):
      xs, ys = [800.0, 900.0, 1000.0, 1100.],  [0., 1., 1., 0.]
      verts.append(list(zip(xs, ys)))
  
  # Z position (i.e. depth at which the XY plot is drawn)
  zs = [0,1,2,3]
  
  colours = plt.cm.Blues(np.linspace(0.2, 0.8, len(zs)))
  poly = PolyCollection(verts, facecolors = colours )
  ax.add_collection3d(poly, zs=zs, zdir='y')
  
  ax.set_xlabel('X')
  ax.set_xlim3d(800,1150)
  ax.set_ylabel('Y')
  ax.set_ylim3d(0, 4)
  ax.set_zlabel('Z')
  ax.set_zlim3d(0, 1)
  plt.show()
  </code></pre>