title: Adding error bars on top of bar charts in Bokeh
body: <p>I am trying to create bar chart with error bars on top. I looked at the following <a href="https://stackoverflow.com/questions/29166353/how-do-you-add-error-bars-to-bokeh-plots-in-python">answer</a> to generate such view. My code works until I do <code>p.line(y_err_x, y_err_y, color="black" )</code> presumably due to <code>x</code> axis indexing as I get the following error: <code>Unable to get property 'A' of undefined or null reference</code></p>
  
  <p>What is the appropriate use? Thanks in advance!</p>
  
  <pre><code>from bokeh.io import show, output_notebook
  from bokeh.models import ColumnDataSource
  from bokeh.plotting import figure
  from bokeh.transform import factor_cmap
  
  output_notebook()
  groups= ['A', 'B', 'C', 'D']
  counts = [5, 3, 4, 2]
  yerr = [1,2,3,4]
  
  source = ColumnDataSource(data=dict(groups=groups, counts=counts))
  
  p = figure(x_range=groups, plot_height=350, toolbar_location=None, title="Values")
  p.vbar(x='groups', top='counts', width=0.9, source=source, legend="groups",
         line_color='white', fill_color=factor_cmap('groups', palette=["#962980","#295f96","#29966c","#968529"], 
                                                    factors=groups))
  
  y_err_x = []
  y_err_y = []
  for px, py, err in zip(groups, counts, yerr):
      y_err_x.append((px, px))
      y_err_y.append((py - err, py + err))
  
  p.line(y_err_x, y_err_y, color="black" )
  
  p.xgrid.grid_line_color = None
  p.legend.orientation = "horizontal"
  p.legend.location = "top_center"
  
  show(p)
  </code></pre>