title: How to use nest in d3 to group using the 'month' as key, but my csv file contains the whole date?
body: <pre><code>           var data = d3.nest()
            .key(function(d) { return d.date;})
            .rollup(function(d) { 
             return d3.sum(d, function(g) {return g.something; });
            }).entries(csv_data);
  </code></pre>
  
  <p>using this code above I can group by date which is in the format yyyy-mm-dd , but I want to group using the month as key. How do I do this ? </p>