title: Date order in bokeh plot with pandas dataframe
body: <p>I have a pandas dataframe with 4 different scores per day:</p>
  
  <pre><code>import pandas as pd 
  df = pd.DataFrame( {u'Date' : ['2014-08-15', '2014-08-16', '2014-08-17', '2014-08-18', '2014-08-19', '2014-08-20',
                                 '2014-08-15', '2014-08-16', '2014-08-17', '2014-08-18', '2014-08-19', '2014-08-20',
                                 '2014-08-15', '2014-08-16', '2014-08-17', '2014-08-18', '2014-08-19', '2014-08-20',
                                 '2014-08-15', '2014-08-16', '2014-08-17', '2014-08-18', '2014-08-19', '2014-08-20'],
                      u'Score': [-0.13, -0.25, 0.1, -0.3, -0.1, 0.1,
                                 -0.1, 0.1, -0.05, 0.2, -0.13, -0.25,
                                 0.2, 0.5, 0.4, -0.2, -0.25, 0.1,
                                 -0.3, 0.1, 0.25, 0.05, -0.05, 0.2]})
      df.Date = pd.to_datetime(df.Date,format='%Y-%m-%d')
  </code></pre>
  
  <p>I would like to plot the daily average of the score. I'm trying with:</p>
  
  <pre><code>from numpy import *
  from bokeh.plotting import *
  output_file('question.html')
  figure(x_axis_type="datetime", plot_width=800, plot_height=500)
  grouped = df.groupby("Date")
  score = grouped["Score"]
  avg = score.mean()
  print dates
  dates = asarray(grouped.groups.keys())
  hold(True)
  line(dates, avg, line_color="grey", line_width=8, line_join="round")
  asterisk(dates, avg, line_color="black", size=15)
  hold(False)
  </code></pre>
  
  <p>However, this code leads to a disordered dates array:</p>
  
  <pre><code>print dates
  ['2014-08-15T02:00:00.000000000+0200' '2014-08-16T02:00:00.000000000+0200'
   '2014-08-18T02:00:00.000000000+0200' '2014-08-19T02:00:00.000000000+0200'
   '2014-08-17T02:00:00.000000000+0200' '2014-08-20T02:00:00.000000000+0200']
  </code></pre>
  
  <p>and the line in the plot I obtain (see the image in <a href="http://siteforplot.weebly.com/" rel="nofollow">this link</a>) is accordingly wrong.</p>
  
  <p>Sortering by date with:</p>
  
  <pre><code>df = df.sort(['Date'])
  </code></pre>
  
  <p>does not helps.</p>
  
  <p>On the other hand, if I try to plot the same dataframe but of 4 days instead of 6:</p>
  
  <pre><code>df = pd.DataFrame( {u'Date' : ['2014-08-15', '2014-08-16', '2014-08-17', '2014-08-18',
                                 '2014-08-15', '2014-08-16', '2014-08-17', '2014-08-18',
                                 '2014-08-15', '2014-08-16', '2014-08-17', '2014-08-18',
                                 '2014-08-15', '2014-08-16', '2014-08-17', '2014-08-18'],
                      u'Score': [-0.13, -0.25, 0.1, -0.3,
                                 -0.1, 0.1, -0.05, 0.2,
                                 0.2, 0.5, 0.4, -0.2,
                                 -0.3, 0.1, 0.25, 0.05]})
  </code></pre>
  
  <p>then the dates array is in order and the plot is correct. I don't understand why, but anyway what I want is the plot with 6 or more dates.</p>
  
  <p>I would appreciate if you could please help me. It is clear that I am doing something wrong but I am a beginner in these languages and I have already tried hundreths of solutions, unsuccesfully. Thank you!</p>