amCharts not displaying when the page loads in AngularJs - angularjs

Question

I am using amCharts to display some data and calling this in my init function. The following is my code:
export default class ACtrl {
constructor($scope, $http, $state) {
var sampleCharts = function () {
let sampleChart;
let sampleBarGraph;
let sampleLine;
const writeAlarmsample = data => {
const {
total,
users
} = data;
// Set cumulative percentage
let runningTotal = 0;
users.forEach(user => {
runningTotal += user.assetAvailability;
user.cumulativePercentage = runningTotal / total * 100;
});
sampleChart.dataProvider = users;
sampleChart.write('userAvailability');
sampleChart.validateData();
};
function handleClick(event){
$state.go("app.userdetails", { userID: event.item.category });
}
// Alarm sample
AmCharts.ready(() => {
sampleChart = new AmCharts.AmSerialChart();
sampleChart.categoryField = "assetId";
//add click listener
sampleChart.addListener("clickGraphItem", handleClick);
var yAxis = new AmCharts.ValueAxis();
yAxis.position = "left";
sampleChart.addValueAxis(yAxis);
var yAxis2 = new AmCharts.ValueAxis();
yAxis2.position = "right";
sampleChart.addValueAxis(yAxis2);
sampleBarGraph = new AmCharts.AmGraph();
sampleBarGraph.valueField = "userAvailability";
sampleBarGraph.type = "column";
sampleBarGraph.fillAlphas = 1;
sampleBarGraph.lineColor = "#f0ab00";
sampleBarGraph.valueAxis = yAxis;
sampleChart.addGraph(sampleBarGraph);
sampleLine = new AmCharts.AmGraph();
sampleLine.valueField = "cumulativePercentage";
sampleLine.type = "line";
sampleLine.lineColor = "#cb0044";
sampleLine.valueAxis = yAxis2;
sampleChart.addGraph(sampleLine);
sampleChart.write('userAvailability');
$http.get(constants.LOCAL_HOST+"/dashboard/users")
.then(response => writeAlarmsample(response.data));
});
};
$scope.init = function() {
availabilityCharts();
};
})
}
The charts load fine when I hit the refresh button, but they are not loaded when the page gets loaded for the first time. I also have a refresh button which calls the function to load the charts even that does not load the charts. If I click on one of the chart items it takes me to details page and when I come back to this page the charts do not load. I have to click on refresh again to load the charts. Can anyone let me know what is the reason for this issue and how I can fix it.