title: How to use groupby in pandas to calculate a percentage / proportion total based on a criteria in another column
body: <p>I'm trying to work out how to use the <code>groupby</code> function in pandas to work out the proportions of values per year with a given Yes/No criteria.</p>
  
  <p>For example, I have a dataframe called <code>names</code>:</p>
  
  <pre><code>  Name  Number  Year   Sex Criteria
  0  name1     789  1998  Male      N
  1  name1     688  1999  Male      N
  2  name1     639  2000  Male      N
  3  name2     551  1998  Male      Y
  4  name2     499  1999  Male      Y
  </code></pre>
  
  <p>I can use</p>
  
  <pre><code>namesgrouped = names.groupby(["Sex", "Year", "Criteria"]).sum()
  </code></pre>
  
  <p>to get: </p>
  
  <pre><code>                   Number
  Sex    Year      Criteria
  Male   1998 N        14507
              Y         2308
         1999 N        14119
              Y         2331
  </code></pre>
  
  <p>and so on.  I would like the 'Number Criteria' column to show the % of the total for each gender and year - so instead of N = 14507 and Y = 2308 for 1998 above I'd have N = 86.27% and Y = 13.73%.</p>
  
  <p>Can anyone advise how to do this?</p>