code: 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
      };
  }
docs: @ngdoc directive
  @name chartGroup
  @description
  `chart-group` is used to group columns, for instance to add them to the
  same column for the same x value. Input is a comma separated string with the
  id's of the columns to group.
  
  Restrict To:
  Element
  
  Parent Element:
  c3chart
  
  @param {String} groupValues Comma separated column ids.
  
  {@link http://c3js.org/reference.html#data-groups| c3js docs}
  
  
  @example
  Usage:
  <chart-group group-values="..."/>
  
  Example:
  {@link http://jettro.github.io/c3-angular-directive/#examples}
  
  <c3chart bindto-id="stacked-bar-plot1-chart">
  <chart-column column-id="data 1"
  column-name="Data 1"
  column-color="red"
  column-values="30,200,100,400,150,250"
  column-type="bar"/>
  <chart-column column-id="data 2"
  column-name="Data 2"
  column-color="green"
  column-values="50,20,10,40,15,25"
  column-type="bar"/>
  <chart-group group-values="data 1,data 2"/>
  </c3chart>