JFreeChart get getWidth() returns 0.0 - jfreechart

Question

I' m using the ChartPanel class from JFreeChart and want to know the width of the plot's data area.
The returned value from the method chartPanel.getChartRenderingInfo().getPlotInfo().getDataArea().getWidth() is 0.0d.
I'm using the JFreeChart version 1.5.0.
My suspicion was a thread-problem, which means that the plotting isn't finished, when I call the .getWidth()-method. So I tried to use ChartChange events, but the result is always a width of 0.0 pixels.
Here is a short example class I created to Show the Problem:
The println in the main() gives "xAxisLengthInPixels = 0.0":
package jFreeChartTest;
import java.awt.BorderLayout;
import javax.swing.JFrame;
import org.jfree.chart.ChartFactory;
import org.jfree.chart.ChartPanel;
import org.jfree.chart.JFreeChart;
import org.jfree.chart.event.ChartChangeEvent;
import org.jfree.chart.event.ChartChangeListener;
import org.jfree.chart.plot.PlotOrientation;
import org.jfree.data.xy.XYSeries;
import org.jfree.data.xy.XYSeriesCollection;
public class Panel_Chart extends JFrame implements ChartChangeListener {
private ChartPanel chartPanel = null;
private int width = 1024;
private int height = 820;
public Panel_Chart() {
this.setTitle("XY Plot");
setBounds(0, 0, width, height);
setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
XYSeriesCollection xySeriesCollection = new XYSeriesCollection();
XYSeries testXYSeries = new XYSeries("Test-Series", true);
testXYSeries.add(4,10);
testXYSeries.add(5,20);
testXYSeries.add(6,30);
xySeriesCollection.addSeries(testXYSeries);
boolean includeLegend = true;
boolean useTooltips = true;
boolean useUrls = true;
JFreeChart chart = ChartFactory.createScatterPlot(//createXYLineChart(
"",
"X-Axis",
"Y-Axis",
xySeriesCollection,
PlotOrientation.VERTICAL,
includeLegend,
useTooltips,
useUrls);
this.chartPanel = new ChartPanel(chart);
getContentPane().add(chartPanel, BorderLayout.PAGE_START);
}
public Double getXAxisLengthInPixels(){
return chartPanel.getChartRenderingInfo().getPlotInfo().getDataArea().getWidth();
}
#Override
public void chartChanged(ChartChangeEvent arg0) {
// TODO Auto-generated method stub
System.out.println("ChartChangeEvent:" + arg0);
}
public static void main(String[] args){
Panel_Chart panelChart = new Panel_Chart();
panelChart.setVisible(true);
System.out.println("xAxisLengthInPixels = " + panelChart.getXAxisLengthInPixels());
}
}