d3.sum = function(array, f) {

  var s = 0,

      n = array.length,

      a,

      i = -1;

  if (arguments.length === 1) {

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

  } else {

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

  }

  return s;

};