docs: function ChartColumn () {
      var columnLinker = function (scope, element, attrs, chartCtrl) {
          var column = attrs.columnValues.split(",");
          column.unshift(attrs.columnId);
          chartCtrl.addColumn(column, attrs.columnType, attrs.columnName, attrs.columnColor);
      };
  
      return {
          "require": "^c3chart",
          "restrict": "E",
          "scope": {},
          "replace": true,
          "link": columnLinker
      };
  }