title: matplotlib axes bar plot unable to plot bars next to each other due to indexing x-axis from pandas df
body: <p>I want to plot 2 bars side by side each other but i keep getting an error:</p>
  
  <p><code>ValueError: Cannot shift with no freq</code> </p>
  
  <p>This error occurred when i set my x in the axes.bar to be x-width. </p>
  
  <p>Here is my code:</p>
  
  <pre><code>df.date_1 = pd.to_datetime(df.date_1)
  df_percent.date_1 = pd.to_datetime(df_percent.date_1)
  
  df = df.set_index(df['date_1']).sort_index() 
  df_percent = df_percent.set_index(['date_1']).sort_index()
  df_percent = df_percent.reindex(df.index).fillna(0)
  
  fig, ax = plt.subplots(figsize=(10, 8))
  ax.plot( df.index, df.line1,label='line1', c='b')
  ax.plot( df.index, df.line2,label='line2', c='r')
  
  ax2=ax.twinx()
  
  #i added the x-10 to the bar chart that i want to shift to the right
  ax2.bar(df_percent.index, df_after, width=10, alpha=0.1, color='r', label='after')
  ax2.bar(df_percent.index-10, df_before, width=10, alpha=0.1, color='g', label='before')
  </code></pre>
  
  <p>If i do a stacked bar chart it works fine.</p>
  
  <hr>
  
  <pre><code>           date_1                       line1                         line2
  date_1                                                              
  2014-06-01 2014-06-01                        65                       66
  2014-07-01 2014-07-01                        68                       70
  2014-08-01 2014-08-01                        62                       65
  2014-09-01 2014-09-01                        62                       76
  2014-10-01 2014-10-01                        63                       66
  2014-11-01 2014-11-01                        79                       80
  2014-12-01 2014-12-01                        80                       50
  2015-02-01 2015-02-01                        70                       72
  2015-03-01 2015-03-01                        67                       67
  2015-04-01 2015-04-01                        69                       60
  2015-05-01 2015-05-01                        66                       83
             date_1                     before                after
  date_1                                                      
  2014-06-01 2014-06-01                 19.80                15.37
  2014-07-01 2014-07-01                 62.82                44.87
  2014-08-01 2014-08-01                 36.70                27.52
  2014-09-01 2014-09-01                 56.18                34.27
  2014-10-01 2014-10-01                 16.31                10.95
  2014-11-01 2014-11-01                 32.35                14.71
  2014-12-01 2014-12-01                 53.33                26.67
  2015-02-01 2015-02-01                 44.44                17.78
  2015-03-01 2015-03-01                 23.08                23.08
  2015-04-01 2015-04-01                 36.84                15.79
  2015-05-01 2015-05-01                 46.58                13.70
  </code></pre>