title: plot dataframe with two y-axes
body: <p>I have the following dataframe: <br/></p>
  
  <pre><code>   land_cover          1         2         3         4         5         6       size
  0          20  19.558872  6.856950  3.882243  1.743048  1.361306  1.026382  16.520265
  1          30   9.499454  3.513521  1.849498  0.836386  0.659660  0.442690  8.652517 
  2          40  10.173790  3.123167  1.677257  0.860317  0.762718  0.560290  11.925280 
  3          50  10.098777  1.564575  1.280729  0.894287  0.884028  0.887448  12.647710
  4          60   6.166109  1.588687  0.667839  0.230659  0.143044  0.070628  2.160922 
  5         110  17.846565  3.884678  2.202129  1.040551  0.843709  0.673298  30.406541 
  </code></pre>
  
  <p>I want to plot the data in the way that: <br/>
  . land_cover is the x-axis  <br/>
  . cols 1 - 6 should be stacked bar plots per land_cover class (row)  <br/>
  . and the column 'size' should be a second y-axis and could be a simple point symbol for every row and additionally a smooth line connecting the points  <br/> <br/>
  Any ideas?</p>