title: groupby() and second variable
body: <pre><code>iris.groupby('species').describe()
  </code></pre>
  
  <p>I am using this code up there and it provides me a summary of all my variables grouped by species.</p>
  
  <p>I just want to see the sum of one variable, I tried this code <code>iris['sepal_lenght'].groupby('species').describe()</code> but it didn't work.</p>
  
  <p>Anyone got an idea how to solve this? </p>