title: Matplotlib: scatter plot with colormaps for edgecolor but no facecolor
body: <p>I want to have a scatter plot with colormap for edgecolors but no facecolors. 
  When I use <code>facecolor='None'</code>, it does not work.</p>
  
  <pre><code>import numpy as np
  import matplotlib.pyplot as plt
  
  
  N = 50
  x = np.random.rand(N)
  y = np.random.rand(N)
  colors = np.random.rand(N)
  area = np.pi * (15 * np.random.rand(N))**2  # 0 to 15 point radii
  
  plt.scatter(x, y, s=area,c=colors,facecolors='None',cmap="gist_rainbow", alpha=0.5)
  plt.show()
  </code></pre>
  
  <p>Any solution?</p>