title: How to add horizontal line (`Span`) to the Box plot in Bokeh?
body: <p>What I want is to render a <a href="http://bokeh.pydata.org/en/latest/docs/user_guide/charts.html#bar-charts" rel="nofollow noreferrer">bar chart</a> with a <a href="https://stackoverflow.com/questions/28797330/infinite-horizontal-line-in-bokeh">horizontal line</a>, so I can see which values (represented by bars) exceed certain treshold (horisontal line). My data is represented by Pandas dataframe.</p>
  
  <pre><code>from bokeh.plotting import figure, output_file, show
  from bokeh.charts import Bar
  from bokeh.models import Span
  from bokeh.io import save
  from pandas import DataFrame
  
  output_file('tmp_file')
  
  p = figure(plot_width=800, plot_height=600)
  
  rows = [(1,'2004-01-15', 10), (2,'2004-02-21', 15)]
  headers = ['id', 'date', 'value']
  
  df = DataFrame.from_records(rows, columns=headers)
  bar = Bar(df, 'date', values='value', agg='mean')
  
  treshold = 12
  
  hline = Span(location=treshold, dimension='width', line_color='red', line_width=3)
  
  p.renderers.extend([hline, bar])
  save(p)
  </code></pre>
  
  <p>The Exeption I get:</p>
  
  <pre><code>File "/usr/lib/python3.4/site-packages/bokeh/core/properties.py", line 1056, in validate
      raise ValueError("expected an element of %s, got seq with invalid items %r" % (self, invalid))
  ValueError: expected an element of List(Instance(Renderer)), got seq with invalid items [&lt;bokeh.charts.chart.Chart object at 0x7fa0a4534f28&gt;]
  </code></pre>