title: No handles with labels found to put in legend error after plotting two plots on one chart
body: <p>I'm plotting some features month by month and highlighted some of them. Before I added the highlight, the legend can show automatically but now it returns <code>No handles with labels found to put in legend</code> error.</p>
  <p>Example data</p>
  <pre><code>df = pd.DataFrame(np.random.randn(10, 4), columns=list('ABCD'))
  highlight = ['A', 'B']
  </code></pre>
  <pre><code>fig, ax = plt.subplots(figsize=(15, 5))
  plt.plot(df.loc[:, ~df.columns.isin(highlight)], c='gray', alpha=0.5) 
  plt.plot(df.loc[:, df.columns.isin(highlight)]) 
  months = pd.date_range('2019-04-01','2019-08-01', freq='MS').strftime(&quot;%Y-%m&quot;).tolist()
  plt.legend(loc='center left', bbox_to_anchor=(1.0, 0.5))
  z = ax.set_xticklabels(months, rotation=45)
  </code></pre>
  <p><a href="https://i.stack.imgur.com/f6amW.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/f6amW.png" alt="enter image description here" /></a></p>
  <p>I guess plotting two plots on one chart caused this but don't know how to fix it. I don't want to manually specify legends.</p>