title: unstacking in a pandas dataframe and plotting the results
body: <p>I have a pandas <code>df</code> like this:</p>
  
  <pre><code>     algo   pairings        mean       stdev
  0   algo1          1    0.289224    0.109537
  1   algo1          3    0.301253    0.024995
  2   algo1          4    0.323887    0.041857
  3   algo2          1    0.289440    0.018899
  4   algo2          3    0.296124    0.023745
  ...
  </code></pre>
  
  <p>and I'd like to plot different lines for each "algorithm" mean in <code>algo</code>, and the error bars using <code>stdev</code>. I've tried the following, but it doesn't work:</p>
  
  <pre><code>df.unstack('algo').plot(yerr="stdev")
  </code></pre>
  
  <p>It tells me <code>KeyError: 'Requested level (algo) does not match index name (None)'</code></p>