docs: function ChartGridOptional() {
      var gridLinker = function (scope, element, attrs, chartCtrl) {
          var axisId = attrs.axisId;
          var value = attrs.gridValue;
          var text = attrs.gridText;
          var gridClass = attrs.gridClass;
          var position = attrs.position;
  
          chartCtrl.addGridLine(axisId, value, text, gridClass, position);
      };
  
      return {
          "require": "^c3chart",
          "restrict": "E",
          "scope": {},
          "replace": true,
          "link": gridLinker
      };
  }