title: MatPlotLib : Changing tick intervals when x axis values are dates
body: <p>I want to display data from a data frame where the index are dates. I want to have a tick per month.
  I want the ticks to be : 
  -jan.
  -feb.
  ...
  -nov.
  -dec.</p>
  
  <p>By default the ticks are :
  - jan.
  -march
  -may
  ... 
  (does'nt display every month)</p>
  
  <p>I have imposed a month format for the ticks and a 45 degrees rotation. I have try :</p>
  
  <pre class="lang-py prettyprint-override"><code>ax1.locator_params(nbins=12, axis='x')
  </code></pre>
  
  <p>That is supposed to make 12 ticks but noting is changing.</p>