title: Change table to tall format using panda (UNPIVOT)
body: <p>I have a table like this</p>
  
  <pre><code>   user         company company2 company3 company4
      1           Mac     Lenovo    Hp      null              
      2           Mac       MSI     Sony                          
  </code></pre>
  
  <p>And using pandas I would like it to be </p>
  
  <pre><code>     user    company
       1          Mac
       1          Lenovo
       1          Hp
       2         Mac
  </code></pre>
  
  <p>and so on
  Here I tried it but didnt work with pandas pivot.</p>
  
  <pre><code>dataframe = pd.read_csv('data.csv')
  dataframe.fillna(value='', inplace=True)
  #dataframe.pivot(index='user', columns='company')
  </code></pre>
  
  <p>Above code doesnt work and gives error. </p>