title: Wrong overlap in bar3d plot
body: <p>I've made this 3d bar plot, but I've found a wrong overlap in some bars, as I show in the image below with green circles:<a href="https://i.stack.imgur.com/vQgUS.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/vQgUS.png" alt="enter image description here"></a></p>
  
  <p>The plot is made by:</p>
  
  <pre><code>import matplotlib.pyplot as plt
  import numpy as np
  from mpl_toolkits.mplot3d.axes3d import Axes3D
  import matplotlib.colors as colors
  
  fig = plt.figure(figsize=(10,8))
  ax = fig.add_subplot(111, projection='3d')    
  matrix = np.array([
  [84 80 68 56 60 44 55 39 27 29]
  [82 67 63 44 47 33 22 19  9  2]
  [53 61 48 34  0 16  0  0  0  0]
  [48 25  0  0  0  0  0  0  0  0]])
  
  len_x, len_y = matrix.shape
  _x = np.arange(len_x)
  _y = np.arange(len_y)
  
  xpos, ypos = np.meshgrid(_x, _y)
  xpos = xpos.flatten('F')
  ypos = ypos.flatten('F')
  zpos = np.zeros_like(xpos)
  
  dx = np.ones_like(zpos)
  dy = dx.copy()
  dz = matrix.flatten()
  
  cmap=plt.cm.magma(plt.Normalize(0,100)(dz))
  
  ax.bar3d(xpos+0.32, ypos-0.3, zpos, dx-0.6, dy-0.1, dz, zsort='max', color=cmap)
  
  ax.set_xlabel('x')
  ax.set_xticks(np.arange(len_x+1))
  ax.set_xticklabels(['1000','500','100','50','0'])
  ax.set_xlim(0,4)
  ax.set_ylabel('y')
  ax.set_yticks(np.arange(len_y+1))
  ax.set_yticklabels(['0.5','1.','1.5','2.','2.5','3.','3.5','4.','4.5','5.'])
  ax.set_ylim(-0.5,10)
  ax.set_zlabel('z')
  ax.set_zlim(0,100)
  ax.view_init(ax.elev, ax.azim+100)
  </code></pre>
  
  <p>Is it a bug? Why some bars are badly overlapping?
  I'm using matplotlib version 2.1.0 and anaconda python 3.6.3</p>