1 d3.sum = function(array, f) {

     2   var s = 0,

     3       n = array.length,

     4       a,

     5       i = -1;

     6 

     7   if (arguments.length === 1) {

     8     while (++i < n) if (!isNaN(a = +array[i])) s += a;

     9   } else {

    10     while (++i < n) if (!isNaN(a = +f.call(array, array[i], i))) s += a;

    11   }

    12 

    13   return s;

    14 };