title: Plot with non-numerical data on x axis (for ex., dates)
body: <p>I'd like to plot numerical data against non numerical data, say something like this:</p>
  
  <pre><code>import matplotlib.pyplot as pl
  x=['a','b','c','d']
  y=[1,2,3,4]
  pl.plot(x,y)
  </code></pre>
  
  <p>However, with matplotlib plot packages you get a warning that the data is not float (ValueError: invalid literal for float(): a). </p>
  
  <p>In their <a href="http://matplotlib.sourceforge.net/faq/howto_faq.html#skip-dates-where-there-is-no-data">'How-to'</a>, they suggest to put first the numerical data on the x axis and then format it.
  Is there a way to do it directly (as above)?</p>