title: How to convert dataframe new format?
body: <p>I have a  dataframe     :</p>
  
  <pre><code>import pandas as pd
  
  df = pd.DataFrame({'cell': ['A1', 'A2', 'B1', 'A3', 'B2', 'B3', 'A4', 'B4'],
                     'site': ['A', 'A', 'B', 'A', 'B', 'B', 'A', 'B']})
  
  &gt;&gt;&gt; df
  
    cell site
  0   A1    A
  1   A2    A
  2   B1    B
  3   A3    A
  4   B2    B
  5   B3    B
  6   A4    A
  7   B4    B
  </code></pre>
  
  <p>I want to get new format    :</p>
  
  <pre><code>A
  A1
  A2
  A3
  A4
  B
  B1
  B2
  B3
  B4
  </code></pre>
  
  <p>Now I want to convert back this result ,but i am not able to do conversion.</p>