Hey, im currently taking a starter course in java and we have been given the task of making a small program with GUI that demonstrates various sorting methods. As a "pre-task" to this project we've been asked to create a program that can make and draw a graph based on a random array with given number of rows, and max/min-values.
Here is what the finished program should look like:


I'm currently having some problems with the drawing part, because once my gui has been created it seems hard to update a panel within the GUI when i hit the draw button.

The empty panel which im having problems getting to update when i click the "draw graph" button. I've called it graphWindow in the picture, but in the code below i think its called grafVindu


I've currently got three classes:
a class called Filler, this one handles creation and filling of the random array.
Java Code: 
import javax.swing.JFrame;
import java.util.Random;

public class Filler {
    int rader=0;
    int[] tabell=null;
    Graf grafz=null;
    public Filler(int a, int b, int c){
        rader=a;
        Random rnd = new Random();
        tabell= new int[a];

        for(int i=0;i<tabell.length -1;i++){
            tabell[i]=rnd.nextInt((c - b))+b;
        }
        grafz=new Graf(tabell);
    }
    public Graf getGraf(){
        return grafz;
    }

}
a class called Graf (=graph)
Java Code: 
import java.awt.Color;
import java.awt.Graphics;
import javax.swing.JPanel;
import javax.swing.JFrame;
import java.util.Random;
public class Graf extends JPanel
{
    int rader=0;
    int[] tabell=null;
    public Graf(int[] b){
        tabell=b;
    }

    public void paintComponent(Graphics g)
    {
        super.paintComponent(g);

        int width = getWidth();
        int height = getHeight();
        int width2 = width / (tabell.length-1);

        for (int i = 0; i < tabell.length-1; i++)
        {
            g.drawRect(i * width2, height - tabell[i], width2, tabell[i]);
        }
    }

}
a class called GUItest, which is mostly designed and generated by Swing with the exception of the actionlistener.
Java Code: 
import javax.swing.JOptionPane;
public class GUItest extends javax.swing.JFrame {

    /** Creates new form GUItest */

    public GUItest() {
        initComponents();
    }

    /** This method is called from within the constructor to
     * initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is
     * always regenerated by the Form Editor.
     */
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">                          
    private void initComponents() {

        jButton1 = new javax.swing.JButton();
        jLabel1 = new javax.swing.JLabel();
        jLabel2 = new javax.swing.JLabel();
        jLabel3 = new javax.swing.JLabel();
        a = new javax.swing.JTextField();
        b = new javax.swing.JTextField();
        c = new javax.swing.JTextField();
        grafVindu = new javax.swing.JPanel();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        jButton1.setText("Draw graph");
        jButton1.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jButton1ActionPerformed(evt);
            }
        });

        jLabel1.setText("Number of rows:");

        jLabel2.setText("Min value:");

        jLabel3.setText("max value:");

        grafVindu.setBackground(new java.awt.Color(255, 255, 255));
        grafVindu.setLayout(new javax.swing.BoxLayout(grafVindu, javax.swing.BoxLayout.LINE_AXIS));

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
                        .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING, false)
                            .addComponent(a, javax.swing.GroupLayout.Alignment.TRAILING)
                            .addComponent(jLabel1, javax.swing.GroupLayout.Alignment.TRAILING, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
                        .addGap(55, 55, 55)
                        .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                            .addComponent(jLabel2)
                            .addComponent(b, javax.swing.GroupLayout.PREFERRED_SIZE, 61, javax.swing.GroupLayout.PREFERRED_SIZE))
                        .addGap(70, 70, 70)
                        .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                            .addComponent(c, javax.swing.GroupLayout.PREFERRED_SIZE, 64, javax.swing.GroupLayout.PREFERRED_SIZE)
                            .addComponent(jLabel3))
                        .addGap(75, 75, 75)
                        .addComponent(jButton1))
                    .addComponent(grafVindu, javax.swing.GroupLayout.DEFAULT_SIZE, 464, Short.MAX_VALUE))
                .addContainerGap())
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addGroup(layout.createSequentialGroup()
                        .addGap(27, 27, 27)
                        .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                            .addComponent(jLabel1)
                            .addComponent(jLabel2)
                            .addComponent(jLabel3))
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                        .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                            .addComponent(a, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                            .addComponent(b, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                            .addComponent(c, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)))
                    .addGroup(layout.createSequentialGroup()
                        .addGap(38, 38, 38)
                        .addComponent(jButton1)))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addComponent(grafVindu, javax.swing.GroupLayout.DEFAULT_SIZE, 331, Short.MAX_VALUE)
                .addContainerGap())
        );

        pack();
    }// </editor-fold>                        

    private void jButton1ActionPerformed(java.awt.event.ActionEvent evt)                                         
    {                                             
                int rader, minVerdi, maxVerdi;
        try
        {
            rader = Integer.parseInt(
                    this.a.getText());
        } catch (Exception e)
        {
            JOptionPane.showMessageDialog(this, "uglyldig rad-verdi", "Error", JOptionPane.ERROR_MESSAGE);
            return;
        }
        try
        {
            minVerdi = Integer.parseInt(
                    this.b.getText());
        } catch (Exception e)
        {
            JOptionPane.showMessageDialog(this, "uglyldig min-verdi", "Error", JOptionPane.ERROR_MESSAGE);
            return;
        }
        try
        {
            maxVerdi = Integer.parseInt(
                    this.c.getText());
        } catch (Exception e)
        {
            JOptionPane.showMessageDialog(this, "uglyldig max-verdi", "Error", JOptionPane.ERROR_MESSAGE);
            return;
        }
        Filler filler = new Filler(rader,minVerdi,maxVerdi);
        
        grafVindu=filler.getGraf();   

    /**
    * @param args the command line arguments
    */
    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new GUItest().setVisible(true);
            }
        });
    }

    // Variables declaration - do not modify                     
    private javax.swing.JTextField a;
    private javax.swing.JTextField b;
    private javax.swing.JTextField c;
    public javax.swing.JPanel grafVindu;
    private javax.swing.JButton jButton1;
    private javax.swing.JLabel jLabel1;
    private javax.swing.JLabel jLabel2;
    private javax.swing.JLabel jLabel3;
    // End of variables declaration                   

}
If i modify my code to open a new window rather than try to update the empty panel the graph draws just fine. So, any ideas on how I can make the graph inside the GUI? Thanks in advance