title: set ticks with logarithmic scale
body: <p>It seems that the <code>set_xticks</code> is not working in log scale:</p>
  
  <pre><code>from matplotlib import pyplot as plt
  fig1, ax1 = plt.subplots()
  ax1.plot([10, 100, 1000], [1,2,3])
  ax1.set_xscale('log')
  ax1.set_xticks([20, 200, 500])
  plt.show()
  </code></pre>
  
  <p>is it possible?</p>