code: function(idx, label, opt) {
  
          var labels = this.labels();
  
          idx = (isFinite(idx) && idx !== null) ? (idx | 0) : 0;
          if (idx < 0) idx = labels.length + idx;
  
          // getter
          if (arguments.length <= 1) return this.prop(['labels', idx]);
          // setter
          return this.prop(['labels', idx], label, opt);
      }
docs: Labels API A convenient way to set labels. Currently set values will be mixined with `value` if used as a setter.