title: axes.fmt_xdata in matplotlib not being called
body: <p>I'm trying to format my X axis dates in a Django application where I'm returning the graph in memory in a response object. I followed the same example that I already use in an ipython notebook, and do this:</p>
  
  <pre><code>def pretty_date(date):
      log.info("HELLO!")
      return date.strftime("%c")
  
  def image_calls(request):
      log.info("in image_loadavg")
  
      datetimes = []
      calls = []
      for m in TugMetrics.objects.all():
          datetimes.append(m.stamp)
          calls.append(m.active_calls)
  
      plt.plot(datetimes, calls, 'b-o')
      plt.grid(True)
      plt.title("Active calls")
      plt.ylabel("Calls")
      plt.xlabel("Time")
  
      fig = plt.gcf()
      fig.set_size_inches(8, 6)
      fig.autofmt_xdate()
  
      axes = plt.gca()
      #axes.fmt_xdata = mdates.DateFormatter("%w %H:%M:%S")
      axes.fmt_xdata = pretty_date
  
      buf = io.BytesIO()
      fig.savefig(buf, format='png', dpi=100)
      buf.seek(0)
      return HttpResponse(buf, content_type='image/png')
  </code></pre>
  
  <p>The graph is returned but I seem to have no control over how to X axis looks, and my HELLO! log is never called. Note that the m.stamp is a datetime object.</p>
  
  <p>This works fine in ipython notebook, both running matplotlib 1.4.2. </p>
  
  <p>Help appreciated.</p>