docs: function ChartGroup () {
      var groupLinker = function (scope, element, attrs, chartCtrl) {
          var group = attrs.groupValues.split(",");
          chartCtrl.addGroup(group);
      };
  
      return {
          "require": "^c3chart",
          "restrict": "E",
          "scope": {},
          "replace": true,
          "link": groupLinker
      };
  }