title: Bokeh plot with datetime period (month-year) on x-axis
body: <p>I am new to bokeh/pandas and trying to plot a trend line by using month-year for  x-axis and integer values for y-axis.</p>
  
  <p>My data looks like below:   </p>
  
  <pre><code>year_month  emp_count
  0   2015-09     1450425
  1   2015-10     3093811
  2   2015-11     3316241
  3   2015-12     3308658
  4   2016-01     3402191
  </code></pre>
  
  <p>To plot using bokeh I am converting both columns to ndarray. When i convert year-month column to ndarray, it shows each value as a Period. I have used to_period('M') method to get year_month out of a date column. </p>
  
  <pre><code>    temp_df.year_month.values
  &gt;&gt;output
      array([Period('2015-09', 'M'), Period('2015-10', 'M'),
             Period('2015-11', 'M'), Period('2015-12', 'M'),
             Period('2016-01', 'M'), Period('2016-02', 'M'),
  </code></pre>
  
  <p>So when i plot using this data, i get following error: </p>
  
  <blockquote>
    <p>TypeError: Object of type 'Period' is not JSON serializable</p>
  </blockquote>
  
  <p>To avoid this error i converted year_month column type to string but i still get the same error. My complete code looks like below:</p>
  
  <pre><code>temp_df.year_month = temp_df.year_month.astype(str)
  output_file('trend1.html')
  p = figure(title='Employee trend', 
             plot_width=800, 
             plot_height=350,
             x_axis_label='Month-Year', y_axis_label='No of Employees', 
            x_axis_type='datetime')
  
  p.line(x= temp_df.year_month, 
         y = temp_df.emp_count)
  
  show(p)
  </code></pre>
  
  <p>Does anyone know how to plot year-month on x-axis using bokeh?</p>