title: Pandas groupby object in legend on plot
body: <p>I am trying to plot a pandas <code>groupby</code> object using the code <code>fil.groupby('imei').plot(x=['time'],y = ['battery'],ax=ax, title = str(i))</code> </p>
  
  <p>The problem is the plot legend lists <code>['battery']</code>as the legend value. Given it's drawing a line for each item in the <code>groupby</code> object, it makes more sense to plot those values in the legend instead. However I'm not sure how to do that. Any help would be appreciated.</p>
  
  <p>Data</p>
  
  <pre><code>                 time             imei  battery_raw
  0 2016-09-30 07:01:23  862117020146766        42208
  1 2016-09-30 07:06:23  862117024146766        42213
  2 2016-09-30 07:11:23  862117056146766        42151
  3 2016-09-30 07:16:23  862117995146745        42263
  4 2016-09-30 07:21:23  862117020146732        42293
  </code></pre>
  
  <p>Full code </p>
  
  <pre><code>for i in entity:
      fil = df[(df['entity_id']==i)]
      fig, ax = plt.subplots(figsize=(18,6))
      fil.groupby('imei').plot(x=['time'],y = ['battery'],ax=ax, title = str(i))  
      plt.legend(fil.imei)
      plt.show()
  </code></pre>
  
  <p>Current plot </p>
  
  <p><a href="https://i.stack.imgur.com/rDNvf.png" rel="noreferrer"><img src="https://i.stack.imgur.com/rDNvf.png" alt="enter image description here"></a></p>