Charts - duplicate children added when sorting the chart data

I want to sort the values of a XYChart based on the X value, because the data comes unsorted.

I get:

Caused by: java.lang.IllegalArgumentException: Children: duplicate children added: parent = Group@158fd70[styleClass=plot-content]
at javafx.scene.Parent$1.onProposedChange(Parent.java:307)
at com.sun.javafx.collections.VetoableObservableList.add(VetoableObservableList.java:165)
at com.sun.javafx.collections.ObservableListWrapper.add(ObservableListWrapper.java:144)
at javafx.scene.chart.LineChart.dataItemAdded(LineChart.java:221)
at javafx.scene.chart.XYChart.dataItemsChanged(XYChart.java:474)
at javafx.scene.chart.XYChart.access$2500(XYChart.java:72)
at javafx.scene.chart.XYChart$Series$1.onChanged(XYChart.java:1462)
at com.sun.javafx.collections.ListListenerHelper$SingleChange.fireValueChangedEvent(ListListenerHelper.java:134)
at com.sun.javafx.collections.ListListenerHelper.fireValueChangedEvent(ListListenerHelper.java:48)
at com.sun.javafx.collections.ObservableListWrapper.callObservers(ObservableListWrapper.java:97)
at com.sun.javafx.collections.ObservableListWrapper.sort(ObservableListWrapper.java:403)
at javafx.collections.FXCollections.sort(FXCollections.java:491)


See this example:


import javafx.application.Application;
import javafx.collections.FXCollections;
import javafx.collections.ObservableList;
import javafx.scene.Scene;
import javafx.scene.chart.LineChart;
import javafx.scene.chart.NumberAxis;
import javafx.scene.chart.XYChart;
import javafx.stage.Stage;

import java.util.Comparator;

public class TestApp3 extends Application {


    @Override
    public void start(Stage stage) throws Exception {

        NumberAxis xAxis = new NumberAxis();
        NumberAxis yAxis = new NumberAxis();

        LineChart<Number, Number> lineChart = new LineChart<Number, Number>(xAxis, yAxis);

        XYChart.Series<Number, Number> series = new XYChart.Series<Number, Number>();
        series.setData(list);

        lineChart.getData().add(series);

        FXCollections.sort(list, new Comparator<XYChart.Data<Number, Number>>() {
            @Override
            public int compare(XYChart.Data<Number, Number> o1, XYChart.Data<Number, Number> o2) {
                return Integer.compare(o1.getXValue().intValue(), o2.getXValue().intValue());
            }
        });

        Scene scene = new Scene(lineChart);
        stage.setScene(scene);
        stage.show();
    }

    private ObservableList<XYChart.Data<Number, Number>> list = FXCollections.observableArrayList(new XYChart.Data<Number, Number>(4, 1), new XYChart.Data<Number, Number>(1, 1), new XYChart.Data<Number, Number>(2, 2), new XYChart.Data<Number, Number>(3, 3));

    public static void main(String[] args) {
        launch(args);
    }
}




If I sort before I add the series, no error output is given at all, but the stage doesn't show up.