title: How can I create a DataFrame with feature, value and index of not missing data (not NaN's)?
body: <p>I have the following Data Frame</p>
  <pre><code>data = {'first_set_of_numbers':  [3,9,6,np.nan],
          'second_set_of_numbers': [np.nan,13,np.nan,np.nan]
         }
  df = pd.DataFrame(data,columns=['first_set_of_numbers','second_set_of_numbers'], index=['A','B','C','D'])
  df
  </code></pre>
  <p><a href="https://i.stack.imgur.com/1IHko.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/1IHko.png" alt="enter image description here" /></a></p>
  <p>How can I now get a new Data Frame that shows all the not missing values with their accoring feature and index? It should look like this:</p>
  <p><a href="https://i.stack.imgur.com/DGbrE.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/DGbrE.png" alt="enter image description here" /></a></p>