title: Bokeh patches plot with dates as x-axis shifts the ticks one to the right
body: <p>I'm trying to adapt the brewer example (<a href="http://docs.bokeh.org/en/latest/docs/gallery/stacked_area.html" rel="nofollow noreferrer">http://docs.bokeh.org/en/latest/docs/gallery/stacked_area.html</a>) to my needs. One of the things I'd like is to have dates at the x-axis. I did the following:</p>
  
  <pre><code>timesteps = [str(x.date()) for x in pd.date_range('1950-01-01', '1951-07-01', freq='MS')]
  p = figure(x_range=FactorRange(factors=timesteps), y_range=(0, 800))
  p.xaxis.major_label_orientation = np.pi/4
  </code></pre>
  
  <p>as an adaptation of the previous line</p>
  
  <pre><code>p = figure(x_range=(0, 19), y_range=(0, 800))
  </code></pre>
  
  <p>The dates are displayed, but the first date 1950-01-01 sits at x=1. How can I shift it to x=0? The first real data points I have are for that date and therefore should be displayed together with that date and not one month later.</p>
  
  <p><img src="https://imgur.com/qHRga6C.png" alt="Graphical explanation"></p>