title: mdates.DateLocator() raising NotImplemented error
body: <p>I am still working on getting nice rounded datatime in xticks:
  <a href="https://stackoverflow.com/questions/46550153/rounding-datetimes-in-xticks-matplotlib">rounding datetimes in xticks matplotlib</a></p>
  
  <p>So the answer was fine, and I could use</p>
  
  <pre><code>plt.gca().xaxis.set_major_locator(mdates.HourLocator()) 
  </code></pre>
  
  <p>to get round hours in the xticks.</p>
  
  <p>However, if I simply do</p>
  
  <pre><code>plt.gca().xaxis.set_major_locator(mdates.DateLocator()) 
  </code></pre>
  
  <p>it gives:</p>
  
  <pre><code>  File "/python/anaconda-2.7.11-64/lib/python2.7/site-packages    /matplotlib/ticker.py", line 1289, in __call__
      raise NotImplementedError('Derived must override')
  NotImplementedError: Derived must override
  </code></pre>
  
  <p>Is it really impossible to change HourLocator() to DateLocator() ? 
  I am using
  <code>matplotlib.__version__'2.0.2'</code></p>