title: TimeSeries in Bokeh using a dataframe with index
body: <p>I'm trying to use Bokeh to plot a <code>Pandas</code> dataframe with a <code>DateTime</code> column containing years and a numeric one. If the <code>DateTime</code> is specified as <code>x</code>, the behaviour is the expected (years in the x-axis). However, if I use <code>set_index</code> to turn the <code>DateTime</code> column into the index of the dataframe and then only specify the <code>y</code> in the <code>TimeSeries</code> I get time in milliseconds in the x-axis. A minimal example</p>
  
  <pre><code>import pandas as pd
  import numpy as np
  from bokeh.charts import TimeSeries, output_file, show
  
  output_file('fig.html')
  test = pd.DataFrame({'datetime':pd.date_range('1/1/1880', periods=2000),'foo':np.arange(2000)})
  fig = TimeSeries(test,x='datetime',y='foo')
  show(fig)
  
  output_file('fig2.html')
  test = test.set_index('datetime')
  fig2 = TimeSeries(test,y='foo')
  show(fig2)
  </code></pre>
  
  <p>Is this the expected behaviour or a bug? I would expect the same picture with both approaches.</p>
  
  <p>Cheers!!</p>