title: Pygal plots involving datetime throw error with dates prior to 1970
body: <p>Trying out the <a href="http://www.pygal.org/en/stable/documentation/types/xy.html?highlight=dateline%20#datetime" rel="nofollow noreferrer">sample code</a> for an XY plot in pygal involving datetime or date, any dates prior to 1970 cause this traceback:</p>
  
  <pre><code>Traceback (most recent call last):
    File "C:/Users/***/dt_test.py", line 30, in &lt;module&gt;
      datetimeline.render()
    File "C:\Python\Python36\lib\site-packages\pygal\graph\public.py", line 52, 
  in render
      self.setup(**kwargs)
    File "C:\Python\Python36\lib\site-packages\pygal\graph\base.py", line 217, 
  in setup
  self._draw()
    File "C:\Python\Python36\lib\site-packages\pygal\graph\graph.py", line 924, 
  in _draw
      self._compute_x_labels()
    File "C:\Python\Python36\lib\site-packages\pygal\graph\dual.py", line 61, 
  in _compute_x_labels
      self._x_labels = list(zip(map(self._x_format, x_pos), x_pos))
    File "C:\Python\Python36\lib\site-packages\pygal\graph\time.py", line 103, 
  in datetime_to_str
      dt = datetime.utcfromtimestamp(x)
  OSError: [Errno 22] Invalid argument
  </code></pre>
  
  <p>Does anyone else get this behavior? (I'm using PyCharm.) 
  lPerhaps the 'millennium' is returning an unexpected negative number?</p>
  
  <p>(Edit)
  I used the code under "date", running in PyCharm:</p>
  
  <pre><code>from datetime import datetime
  datetimeline = pygal.DateTimeLine(
      x_label_rotation=35, truncate_label=-1,
      x_value_formatter=lambda dt: dt.strftime('%d, %b %Y at %I:%M:%S %p'))
  datetimeline.add("Serie", [
      (datetime(2013, 1, 2, 12, 0), 300),
      (datetime(2013, 1, 12, 14, 30, 45), 412),
      (datetime(2013, 2, 2, 6), 823),
      (datetime(2013, 2, 22, 9, 45), 672)
  ])
  datetimeline.render()
  </code></pre>
  
  <p>... when I change the '2013' to '1969,' I get the Traceback shown above.</p>