[Java] Plott Problem!

import java.awt.*; 
import java.applet.*; 
import java.awt.event.*;

public class koordinatensystem extends Applet implements ActionListener
{
 private TextField a,b,c;
 private Label labela,labelb,labelc;
 double az = 1 ,bz = 1,cz = 1;
 
 public void init()
 {
 
 setLayout(null);
 Color s = new Color (145,240,200);
 setBackground(s);
 
 labela = new Label ("x^3+ ");
 labelb = new Label ("x^2+ ");
 labelc = new Label ("x: ");
 
 a = new TextField (10);
 b = new TextField (10);
 c = new TextField (10);
 
 add(labela);
 add(labelb);
 add(labelc);
 add (a);
 labela.setBounds (10,10,60,20);
 a.setBounds (80,10,60,20);
 a.addActionListener(this);
 add (b);
 labelb.setBounds (10,40,60,20);
 b.setBounds (80,40,60,20);
 b.addActionListener(this);
 add (c);
 labelc.setBounds (10,70,60,20);
 c.setBounds (80,70,60,20);
 c.addActionListener(this);
 
 }
 
 public void actionPerformed(ActionEvent ereignis)
 {
 
 if (ereignis.getSource()==a)
 {
 az = Double.valueOf(a.getText());
 repaint();
 }
 if (ereignis.getSource()==b)
 {
 bz = Double.valueOf(b.getText());
 repaint();
 }
 if (ereignis.getSource()==c)
 {
 cz = Double.valueOf(c.getText());
 repaint();
 }
 }
 
 
 public void paint ( Graphics gr )
 {
 
 for (int x=0;x<=500;x=x+10)
 {
 gr.setColor (Color.lightGray);
 gr.drawLine(0,x,500,x);
 gr.drawLine(x,0,x,500);
 gr.setColor(Color.blue);
 gr.drawLine(x,246,x,254);
 gr.drawLine(246,x,254,x);
 }
 
 
 gr.setColor (Color.blue);
 gr.drawLine (250,0,250,500);
 gr.drawLine (0,250,500,250);
 
 for (double i = -250; i<=250;i++)
 {
 double y = i*i*i*az+i*i*bz+i*cz;
 
 int xpunkt = (250+(int)i)*10;
 int ypunkt = (250-(int)y)*10;
 gr.drawLine(xpunkt,ypunkt,xpunkt,ypunkt);
 
 }
 }
}