title: plot pandas dataframe two columns
body: <p>i have a pandas dataframe which has dates as indexes and some columns:
  I would like to plot a line chart with 2 lines (let's say 'ISP.MI' and 'Ctrv'); on the x axis I need the 'Date' </p>
  
  <pre><code>Ticker       ISP.MI  Daily returns        Ctrv  Inv_Am  Giac_Media
  Date                                                                 
  2016-01-01  2.90117            NaN  100.000000     100       100.0   
  2016-01-04  2.80159      -0.034927  196.507301     200       150.0   
  2016-01-05  2.85608       0.019263  300.292610     300       200.0   
  2016-01-06  2.77904      -0.027345  392.081255     400       250.0   
  2016-01-07  2.73206      -0.017050  485.396411     500       300.0   
  2016-01-08  2.72267      -0.003443  583.725246     600       350.0   
  </code></pre>