title: Pivot table using Pandas, add extra row at the end with subtract calculation
body: <p>I have a dataframe df, looks like below:</p>
  
  <pre><code>product    price
  
  prod0       10
  prod1       20
  prod2       30
  prod3       40         
  cashback    30
  </code></pre>
  
  <p>I want to create a pivot table using panda, where margin will be "GrandTot" and extra row will be there at the end "with cashbak"
  how can I get an output like below?</p>
  
  <pre><code>col_name     price     GrandTot
  Product
  prod0         10         10
  prod1         20         20
  prod2         30         30
  prod3         40         40
  cashback      30         30
  GrandTot      100        100
  with cashbak  70         70 
  </code></pre>