title: Add Band object to legend in Bokeh?
body: <p>I want to add a Band to the layout in a plot. Minimal example:</p>
  
  <pre><code>df = pd.DataFrame(dict(x = np.arange(100),
                         y1 = np.random.rand(100),
                         y2 = np.random.rand(100) + 10)
                    )
  p = figure(y_range = (df.y1.min(), df.y2.max()), plot_height = 400)
  source = ColumnDataSource(df)
  band = Band(base='x', lower='y1', upper='y2', source=source)
  p.add_layout(band)
  df.loc[:, "mean_line"] = (df.y1 + df.y2)/2
  p.line(x = df.x, y = df.mean_line, legend = "mean")
  p.legend.click_policy = "hide"
  show(p)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/AHdXL.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/AHdXL.png" alt="example of band plot"></a>
  In the above example, when you click on "mean" in the legend then the mean line will hide itself. However, I can't find a way to hide the "Band" object.</p>
  
  <p>I tried the solution suggested <a href="https://stackoverflow.com/questions/48439142/bokeh-add-legend-to-step-graph">here</a> e.g. 
  <code>li1 = LegendItem(label='red', renderers=[p.renderers[0]])</code>
  etc. except I don't think the "Band" has a renederer in the figure as it's added as a layout?</p>
  
  <p>How would I be able to add the band to the legend so I can hide it as I do with the line?</p>