title: How to add legend inside Python's Bokeh circle plot
body: <p>I have the following code:</p>
  
  <pre><code>from bokeh.plotting import figure, show, output_file
  from bokeh.sampledata.iris import flowers
  
  colormap = {'setosa': 'red', 'versicolor': 'green', 'virginica': 'blue'}
  colors = [colormap[x] for x in flowers['species']]
  
  p = figure(title = "Iris Morphology")
  p.xaxis.axis_label = 'Petal Length'
  p.yaxis.axis_label = 'Petal Width'
  
  p.circle(flowers["petal_length"], flowers["petal_width"],
           color=colors, fill_alpha=0.2, size=10)
  
  output_file("iris.html", title="iris.py example")
  
  show(p)
  </code></pre>
  
  <p>Which produces this plot</p>
  
  <p><a href="https://i.stack.imgur.com/bKmCd.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/bKmCd.jpg" alt="enter image description here"></a></p>
  
  <p>How can I add legend based on the color of the circle to the plot?
  Where: </p>
  
  <ul>
  <li>setosa     : red    </li>
  <li>versicolor : green  </li>
  <li>virginica  : blue</li>
  </ul>