Highcharts is not defined

I made a custom directive in angular angular.module('myModule', []).directive('hcPieChart', function () { return { restrict: 'E', template: '', scope: { title: '@', data: '=' }, link: function (scope, element) { Highcharts.chart(element[0], { chart: { type: 'pie' }, title: { text: scope.title }, plotOptions: { pie: { allowPointSelect: true, cursor: 'pointer', dataLabels: { enabled: true, format: '{point.name}: {point.percentage:.1f} %' } } }, series: [{ data: scope.data }] }); } }; })

and have included

in my index.html . But while running my application I am getting "Highcharts is not defined in directive". Kindly tell me what am I doing wrong?