code: function d3_format_group(value) {
    var i = value.lastIndexOf("."),
        f = i >= 0 ? value.substring(i) : (i = value.length, ""),
        t = [];
    while (i > 0) t.push(value.substring(i -= 3, i + 3));
    return t.reverse().join(",") + f;
  }
docs: Apply comma grouping for thousands.