JavaFx update linechart which is part of fxml does not work - javafx

Question

I am new to JavaFx:
In my FXML I defined a linechart and over a controller I want to change e.g. the title of the chart.
But somehow it is not working.
Here the FXML:
<?xml version="1.0" encoding="UTF-8"?>
<?import javafx.scene.chart.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>
<StackPane maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="400.0" prefWidth="600.0" xmlns="http://javafx.com/javafx/10.0.2-internal" xmlns:fx="http://javafx.com/fxml/1" fx:controller="com.ui.views.view2.viewView2Controller">
<children>
<VBox alignment="CENTER" prefHeight="200.0" prefWidth="100.0">
<children>
<Label text="View 2 Area" />
<Button mnemonicParsing="false" onAction="#nextPane" text="view 1 page" />
<LineChart fx:id="testChart">
<xAxis>
<CategoryAxis side="BOTTOM" />
</xAxis>
<yAxis>
<NumberAxis side="LEFT" />
</yAxis>
</LineChart>
</children>
</VBox>
</children>
</StackPane>
Here the controller Code:
package com.ui.views.view2;
import com.ui.libs.VistaNavigator;
import javafx.event.ActionEvent;
import javafx.fxml.FXML;
import javafx.scene.chart.*;
public class viewView2Controller {
#FXML
private LineChart<String,Number> testChart;
#FXML
private Axis<String> xAxis;
#FXML
private Axis<Number> yAxis;
public viewView2Controller()
{
xAxis = new CategoryAxis(); // we are gonna plot against time
yAxis = new NumberAxis();
xAxis.setLabel("Time/s");
xAxis.setAnimated(false); // axis animations are removed
yAxis.setLabel("Value");
yAxis.setAnimated(false); // axis animations are removed
//creating the line chart with two axis created above
testChart = new LineChart<>(xAxis,yAxis);
testChart.setTitle("my first JavaFX Chart");
testChart.setAnimated(false);
}
public void nextPane(ActionEvent actionEvent) {
VistaNavigator.loadVista(VistaNavigator.viewConfig);
}
}
Whats wrong in my code and what do I need to change that I am able to change the chart title and afterwards with the new recommendation also able to populate values into it. :-)
BR
Dieter