title: matplotlib: scatter plot from string
body: <p>I have data in this form in a text file: </p>
  
  <pre><code>strings  year  avg
  --       --    --
  abc      2012  1854
  abc      2013  2037
  abc      2014  1781
  pqr      2011  1346
  pqr      2012  1667
  xyz      2015  1952
  </code></pre>
  
  <p>I want to make a scatter plot with (distinct) strings on the x-axis, (distinct) year on the y-axis and the size of marker (circle) should be equal to the avg. I am having trouble implementing it in matplotlib, because the scatter function expects a numerical value for x,y (data positions). Because of that, I am unable to assign strings as x and year as y. Do I need to pre-process this data further?</p>