title: pandas groupby two columns and multiply two other columns
body: <p>I have a dataframe grouped by like this;</p>
  
  <pre><code>                      price      quantity   vat
  date      brand
  20-Jun-13 Reebok         7.0         8    2.2
            Adidas        12.0         3    3.8
            Campus         2.5        38    4.2
            Woodlands     23.0         9    7.2
            Boot           3.2        35    3.3
  21-Jun-13 Reebok         7.0         6    2.2
            Adidas        12.0        23    3.8
            Campus         2.5        18    4.2
            Woodlands     23.0        29    7.2
            Boot           3.2        15    3.3
  22-Jun-13 Reebok         5.0         2    3.5
            Adidas        10.0         5    2.8
            Campus         2.0        50    3.5
            Woodlands     25.0         4    6.5
            Boot           2.5        10    2.8
  </code></pre>
  
  <p>How do I groupby <code>'date'</code> and <code>'brand'</code> and multiply <code>'price'</code> and <code>'quantity'</code> to calculate sales?</p>
  
  <p>I've tried;</p>
  
  <p>print <code>data2.groupby(['date','brand'])['price'] * ['quantity']</code></p>
  
  <p>I would like to calculate the total sales by date.</p>