can't update values dynamically

[serenity001]

hello , I'm trying to use ng-charts to create pie-charts , it works fine when the values and labels are static like in the example given but when I try to retrieve the labels and values dynamically from web API and then assign it to $scope.pieChartData an error accurs saying :
Error: b.datasets[0].y is undefined
does anyone know how to fix that problem ?
thank you