Duvida com JPanel e drawImage

import org.jCharts.axisChart.customRenderers.axisValue.renderers.ValueLabelPosition;
import org.jCharts.axisChart.customRenderers.axisValue.renderers.ValueLabelRenderer;
import org.jCharts.chartData.AxisChartDataSet;
import org.jCharts.chartData.ChartDataException;
import org.jCharts.chartData.DataSeries;
//import org.jCharts.imageMap.ImageMap;
import org.jCharts.properties.*;
import org.jCharts.axisChart.*;

import java.awt.image.BufferedImage;
import javax.swing.*;
import java.awt.*;
import java.awt.event.WindowEvent;
import org.jCharts.types.ChartType;

/**
 *
 * @author rodrigo.sm
 */
public class BarDemo extends JFrame {
    
    private JPanel panel;   
    
    public BarDemo() throws ChartDataException, PropertyException {  
        inicializaPanel();
        mostraGrafico();
    }
    
    private void inicializaPanel() throws ChartDataException, PropertyException{
        this.setSize(1024,768);
        this.panel = new JPanel(true);
        this.panel.setSize(1024,768);       
        this.getContentPane().add(this.panel);        

        addWindowListener( new java.awt.event.WindowAdapter()
		{
			public void windowClosing( WindowEvent windowEvent )
			{
				exitForm( windowEvent );
			}
		}
		);
    }
    
    private AxisChart montaGrafico() throws ChartDataException, PropertyException {
        
        String[] xRotulo = {"1","2","3","4","5","6","7"};
        String xTitulo = "Valor";
        String yTitulo = "Y";
        String Titulo = "Teste";
        
            
        DataSeries dataSeries = new DataSeries (xRotulo, xTitulo,yTitulo,Titulo);
        double[][] dados = new double[][]{{10,50,55,22,37,69,82},{50,22,80,64,120,10,40}};
        String[] legenda = {"Barra 1","Barra 2"};
        Paint[] cores = new Paint[]{Color.cyan, Color.RED};
       
        ClusteredBarChartProperties clusteredBarChartProperties = new ClusteredBarChartProperties();
        ValueLabelRenderer valueLabelRenderer = new ValueLabelRenderer( false, false, true, -1 );
        valueLabelRenderer.setValueLabelPosition( ValueLabelPosition.ON_TOP );
        valueLabelRenderer.useVerticalLabels( false );
        clusteredBarChartProperties.addPostRenderEventListener( valueLabelRenderer );

        AxisChartDataSet axisChartDataSet = new AxisChartDataSet(dados,legenda, cores, ChartType.BAR_CLUSTERED,clusteredBarChartProperties);      
       
        dataSeries.addIAxisPlotDataSet(axisChartDataSet); 
        AxisChart axisChart = new AxisChart(dataSeries, new ChartProperties(),  new AxisProperties(), new LegendProperties(),1020,720);                     
        return axisChart;
    }
    
    public void mostraGrafico () throws ChartDataException, PropertyException{
        AxisChart grafico = montaGrafico();
        BufferedImage bufferedImage = new BufferedImage( 1020, 720, BufferedImage.TYPE_INT_RGB );    
        Graphics2D graf = bufferedImage.createGraphics(); 
	grafico.setGraphics2D( bufferedImage.createGraphics() );       
        grafico.render();        
        this.setVisible(true);        
        this.panel.getGraphics().drawImage( bufferedImage, 0, 0, this );        
    }
    private void exitForm( WindowEvent windowEvent ){
		System.exit( 0 );
    }
    
    public static void main( String args[] ) throws ChartDataException, PropertyException{
		new BarDemo();
   }
}