title: Date ticks and rotation in matplotlib
body: <p>I am having an issue trying to get my date ticks rotated in matplotlib. A small sample program is below. If I try to rotate the ticks at the end, the ticks do not get rotated. If I try to rotate the ticks as shown under the comment 'crashes', then matplot lib crashes. </p>
  
  <p>This only happens if the x-values are dates. If I replaces the variable <code>dates</code> with the variable <code>t</code> in the call to <code>avail_plot</code>, the <code>xticks(rotation=70)</code> call works just fine inside <code>avail_plot</code>. </p>
  
  <p>Any ideas? </p>
  
  <pre><code>import numpy as np
  import matplotlib.pyplot as plt
  import datetime as dt
  
  def avail_plot(ax, x, y, label, lcolor):
      ax.plot(x,y,'b')
      ax.set_ylabel(label, rotation='horizontal', color=lcolor)
      ax.get_yaxis().set_ticks([])
  
      #crashes
      #plt.xticks(rotation=70)
  
      ax2 = ax.twinx()
      ax2.plot(x, [1 for a in y], 'b')
      ax2.get_yaxis().set_ticks([])
      ax2.set_ylabel('testing')
  
  f, axs = plt.subplots(2, sharex=True, sharey=True)
  t = np.arange(0.01, 5, 1)
  s1 = np.exp(t)
  start = dt.datetime.now()
  dates=[]
  for val in t:
      next_val = start + dt.timedelta(0,val)
      dates.append(next_val)
      start = next_val
  
  avail_plot(axs[0], dates, s1, 'testing', 'green')
  avail_plot(axs[1], dates, s1, 'testing2', 'red')
  plt.subplots_adjust(hspace=0, bottom=0.3)
  plt.yticks([0.5,],("",""))
  #doesn't crash, but does not rotate the xticks
  #plt.xticks(rotation=70)
  plt.show()
  </code></pre>