code: function(ch, w) {
     if(!ch || ch.length == 0) return Number.MAX_VALUE;
     var arSum = 0;
     for(var i=0, l=ch.length; i<l; i++) {
       var area = ch[i]._area;
       var h = area / w;
       arSum += w > h? w / h : h / w;
     }
     return arSum / l;
   }
docs: /*
  Method: avgAspectRatio
  
  Calculates the average aspect ratio of a group of rectangles.
  
  See also:
  
  <http://en.wikipedia.org/wiki/Aspect_ratio>
  
  Parameters:
  
  ch - An array of nodes.
  w - The fixed dimension where rectangles are being laid out.
  
  Returns:
  
  The average aspect ratio.