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

     2   var n = array.length,

     3       a,

     4       m = 0,

     5       i = -1,

     6       j = 0;

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

     8     while (++i < n) if (d3_number(a = array[i])) m += (a - m) / ++j;

     9   } else {

    10     while (++i < n) if (d3_number(a = f.call(array, array[i], i))) m += (a - m) / ++j;

    11   }

    12   return j ? m : undefined;

    13 };