title: Matplotlib using twinx() - TypeError: 'AxesSubplot' object does not support item assignment
body: <p>I am trying to have a multiple subplots with <code>twinx()</code> for the charts, but met with this error:
  TypeError: 'AxesSubplot' object does not support item assignment
  The code is as following:</p>
  <pre><code>fig, ax = plt.subplots(4,5,figsize=(8,8))
  ax2[0,0]=ax[0,0].twinx()
  </code></pre>
  <p>When I try another way without the arrays:</p>
  <pre><code>fig, ax = plt.subplots(4,5,figsize=(8,8))
  ax2=ax.twinx()
  </code></pre>
  <p>it comes out with another error:</p>
  <blockquote>
  <p>AttributeError: 'numpy.ndarray' object has no attribute 'twinx'</p>
  </blockquote>