title: I applied sum() on a groupby and I want to sort the values of the last column
body: <p>Given the following DataFrame</p>
  
  <pre><code>user_ID  product_id  amount
     1       456          1
     1        87          1
     1       788          3
     1       456          5
     1        87          2
    ...      ...         ...
  </code></pre>
  
  <p>The first column is the ID of the customer, the second is the ID of the product he bought and the 'amount' express if the quantity of the product purchased on that given day (the date is also taken into consideration). a customer can buy many products each day as much as he wants to.
  I want to calculate the total of times each product is bought by the customer, so I applied a <code>groupby</code></p>
  
  <pre><code>df.groupby(['user_id','product_id'], sort=True).sum()
  </code></pre>
  
  <p>now I want to sort the sum of amount in each group.
  Any help?</p>