code: 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
      };
  }
docs: @ngdoc directive
  @name chartLine
  @description
  `chart-line` is used to customize the line chart properties..
  
  Restrict To:
  Element
  
  Parent Element:
  c3chart
  
  @param {String} stepType Step types for step chart: step, step-before and step-after.
  
  {@link http://c3js.org/reference.html#line-step_type| c3js doc}
  
  @param {Boolean} connectNull Should null data point be connected or not.
  
  {@link http://c3js.org/reference.html#bar-width-ratio| c3js doc}
  
  @example
  Usage:
  <chart-line step-type="..." connect-null="..."/>
  Example:
  {@link http://jettro.github.io/c3-angular-directive/#examples}