How can I get ST-BANDUNG to alert undefined?

This my code :

onClick: function(evt) {
  var activePoints = grafik_1.getElementsAtEvent(evt);
  if (activePoints[0]) {
    var chartData = activePoints[0]["_chart"].config.data;
    var idx = activePoints[0]["_index"];
    var dIndex = grafik_1.getDatasetAtEvent(evt)[0]._datasetIndex;

    var label = chartData.labels[idx];
    var value = chartData.datasets[dIndex].data[idx];
    var area = chartData.datasets[dIndex].label[idx];

    grafik_1.update();

    var url = area + " -> " + "label = " + label + " value = " + value;
    console.log(url);
    alert(url);
  }
},