title: melt multiple columns in a dataframe
body: <p>Suppose I have the following dataframe:
  <a href="https://i.stack.imgur.com/lvYbF.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/lvYbF.png" alt="enter image description here"></a></p>
  
  <p>What I want is process the dataframe to the following form:</p>
  
  <p><a href="https://i.stack.imgur.com/nRK4F.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/nRK4F.png" alt="enter image description here"></a></p>
  
  <p>I tried pd.melt function but unfortunately it just does really poor job with multiple columns melting. Any idea please? </p>
  
  <p>Regards</p>