access parent scope attribute in directive's link function through child directive's attribute using bindToController feature

testApp.js

// plunker
// http://plnkr.co/edit/fYTqD8qYPcULRKZ2zqdA

(function() {
  var app1 = angular.module('myApp1', ['myApp2']);
  app1.directive('myApp', function() {
    return {
      scope: {},
      template: '<inside-directive chart-config="vm.chartConfig"></inside-directive>',
      bindToController: true,
      controller: function () {
        this.chartConfig = {
          'test': true
        };
      },
      controllerAs: 'vm'
    };
  });
  
  var app2 = angular.module('myApp2', []);
  app2.directive('insideDirective', function() {
    return {
      scope: {
        chartConfig: '=?'
      },
      bindToController: {
        chartConfig: '=?'
      },
      template: '<b>Hello from inside directive</b>.',
      controller: function () {},
      controllerAs: 'vm',
      link: function (scope, elem, attr, ctrl) {
        elem.append(JSON.stringify(scope.chartConfig))
      }
    };
  });
  
  
})();