title: changing facecolor "alpha" value gives unwanted edges (matplotlib pcolormesh)
body: <p>I need to introduce a non-constant alpha value using pcolormesh (imshow is a priori not a possible substitue because I need to use log scale for the axes -- hence non-regular spacing along each coordinate).</p>
  
  <p>Following <a href="https://stackoverflow.com/questions/32177718/use-a-variable-to-set-alpha-opacity-in-a-colormap">this post</a>, I tried to change a posteriori the alpha value of the faces. However, in the results, I can't get rid of edges that appear. </p>
  
  <p>Here is a minimal example, where I plot a 2D gaussian bump (with very few points), with alpha increasing from the lower left to the upper right corner:</p>
  
  <pre><code>from matplotlib import pyplot as plt
  import numpy as np
  
  # start with coordinates, corresponding meshgrid to compute the "shading" value and 
  # extended coordinate array for pcolormesh (center mesh)
  xx = np.linspace(-4,4,7)
  xmesh, ymesh = np.meshgrid(xx,xx)
  xplot = np.pad(0.5*(xx[1:]+xx[:-1]),1,'reflect',reflect_type="odd") # center &amp; extend
  yy = np.exp(-xx[None,:]**2-xx[:,None]**2) # data to plot
  
  # plot the data
  fig = plt.figure()
  hpc = plt.pcolormesh(xplot, xplot, yy, shading="flat", edgecolor=None)
  plt.gca().set_aspect(1)
  
  # change alpha of the faces: lower-left to upper-right  gradient
  fig.canvas.draw()  # this generate face color array
  colors = hpc.get_facecolor()
  grad = ( (xmesh.ravel()+ymesh.ravel())/2. - xx.min() ) / ( xx.max()-xx.min() )
  colors[:,3] = grad.ravel()  # change alpha
  hpc.set_facecolor(colors)   # update face colors
  fig.canvas.draw()           # make the modification appears
  </code></pre>
  
  <p>The result looks like this: 2D gaussian bump (with very few points), with alpha increasing from the lower left to the upper right corner:
  <img src="https://i.stack.imgur.com/W1DHw.png" alt="The result looks like this: 2D gaussian bump (with very few points), with alpha increasing from the lower left to the upper right corner"></p>
  
  <p>Is it possible to get rid of these edges ? My problem is that I don't even know where it comes from... I tried adding <code>hpc.set_antialiased(True)</code>, <code>hpc.set_rasterized(True)</code>, explicitely adding edges with <code>hpc.set_facecolor(face)</code>, tuning the linewidth to very small values -- none of these worked.</p>
  
  <p>Thanks a lot for your help</p>