title: mplot3d: change default (opposite) shading
body: <p>I was using <code>plot_surface</code> to create a conical structure with up and down parts in 3D, see code below. With viewing angle <code>azim=90.</code> the bottom cone is lighter on the left and darker on the right. One would expect the same for the top cone if the "light source" is coming from the left side. However, the upper cone has opposite shading and remains so for other viewing angles I choose. </p>
  
  <pre><code>import matplotlib.pyplot as plt
  import numpy as np
  
  fig = plt.figure(figsize=(3,4));
  ax = fig.add_subplot(111,projection='3d')
  
  theta = np.linspace(0,2*np.pi,360)
  r = np.linspace(0,1,100)
  T, R = np.meshgrid(theta, r)
  
  X = R * np.cos(T)
  Y = R * np.sin(T)
  Zup = np.sqrt(X**2 + Y**2)
  
  ax.plot_surface(X, Y, Zup, rstride=1, cstride=1, linewidth=0, 
                 antialiased=True,alpha=0.7,color='orange')
  ax.plot_surface(X, Y, -Zup, rstride=1, cstride=1, linewidth=0, 
                 antialiased=True,alpha=0.7,color='orange')
  
  ax.set_axis_off()
  ax.view_init(elev=4., azim=90.)
  ax.dist=6
  
  fig.tight_layout(pad=0.)
  </code></pre>
  
  <p>At certain angle (eg. <code>azim=45</code>) both cones looks homogeneous but I would like them to have some (<em>consistent</em>) lighting. Thanks in advance. </p>
  
  <p><a href="https://i.stack.imgur.com/eMBda.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/eMBda.png" alt="enter image description here"></a></p>