docs: function ChartLine() {
      var lineLinker = function (scope, element, attrs, chartCtrl) {
          var line = {};
          if (attrs.stepType) {
              line.step = line.step || {};
              line.step.type = attrs.stepType;
          }
          if (attrs.connectNull) {
              line.connectNull = (attrs.connectNull === 'true');
          }
          chartCtrl.addLine(line);
      };
  
      return {
          require: '^c3chart',
          restrict: 'E',
          scope: {},
          replace: true,
          link: lineLinker
      };
  }