title: Holoviews - How to create side-by-side bars from dataframe columns?
body: <p>I would like to create a Holoviews bar chart (using Bokeh backend) in which Year is on the X-axis and columns A and B on the Y-Axis. For each Year, I want bars for values from columns A and B to appear next to each other i.e. for Year 2008, I have bars of heights 1 and 3, for year 2009, I have bars 3 and 6 height, and so on. I have tried numerous different ways including the example of <a href="http://holoviews.org/reference/elements/bokeh/Bars.html" rel="nofollow noreferrer">grouped bars</a> in the documentation but can't get it to work. See example below:</p>
  
  <pre><code>%%opts Bars [xrotation=90 width=600 show_legend=False tools=['hover']]
  df=pd.DataFrame({'Year':[2008,2009,2010,2011,2012,2013],
     'A': [1,2,3,4,5,6],'B':[3,6,9,12,15,18]})
  print(df)
  bars = hv.Bars(df, kdims=['Year'], vdims=['A'])
  bars
  </code></pre>
  
  <p>Please help. I am losing my mind!</p>