MorrisJS错误：无法读取未定义的属性“标签”

Question

我正在使用anuglarjs和morris来创建图表。

我的指示是：<​​/ p>

    app.directive('barchart', function() {

    return {

        // required to make it work as an element
        restrict: 'E',
        template: '<div></div>',
        replace: true,
        // observe and manipulate the DOM
        link: function($scope, element, attrs) {

            var data = $scope[attrs.data],
                xkey = $scope[attrs.xkey],
                ykeys= $scope[attrs.ykeys],
                labels= $scope[attrs.labels];

            Morris.Bar({
                element: element,
                data: data,
                xkey: xkey,
                ykeys: ykeys,
                labels: labels
            });

        }

    };
});

我从api获取数据的服务：

services.factory('Temp', function ($resource) {
    return $resource('http://localhost/api/Temp', {}, {
        query: { method: 'GET',  isArray: true}

    })
});

我的控制员：

app.controller('sampleController', ['$scope', 'Temp',
    function($scope, Temp){

    $scope.xkey = 'date';

    $scope.ykeys = ['temp', 'humidity'];

    $scope.labels = ['Temperature', 'Humidity'];

    $scope.myModel = Temp.query();
        console.log($scope.myModel);

}]);

问题是我收到此错误：无法读取未定义的属性'label'。我的api返回一个json数组，如下所示：

[{"date":"20150717105501","temp":"26","humidity":"38"},
{"date":"20150717105401","temp":"26","humidity":"38"},
{"date":"20150717105301","temp":"26","humidity":"37"},
{"date":"20150717105101","temp":"36","humidity":"0"}]

我看不出有什么问题，我是角色的新手。谢谢！