title: Using D3.nest() to create an array of objects by column header
body: <p>I have a CSV file with the format:</p>
  
  <p>time,group1,group2,group3<br>
  0,45,30,30,<br>
  1,30,25,31,<br>
  2,50,45,30,  </p>
  
  <p>I want to structure it as an array of objects such that :<br>
  <code>[{group1:array[3]}, {group2:array[3]},...]</code><br>
  where each <code>array[3]</code> is itself an array of objects that pairs 
  the value in the time column with the value in its respective group column 
  i.e. :<br>
  group1 <code>[{time:0, value:45},{time:1, value:30},...]</code> group2 <code>[{time:0, value:30},...]</code></p>
  
  <p>D3.csv parses by row  and I'm not sure how to iterate through the resulting array of objects with d3.nest or if there's a way to set up the data structure within the d3.csv accessor function. (I apologize if I'm using terms improperly)</p>