java画函数曲线

我做了一个简单的例子，还没有完善好，只是起到一个抛砖引玉的作用，希望你能自己把它补完整。

package test;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.border.LineBorder;

public class DrawFn extends JFrame{

private JTextField txt_interval;
private JTextField txt_c;
private JTextField txt_b;
private JTextField txt_a;

private drawFnPanel panel = new drawFnPanel();
/**
* @param args
*/
public static void main(String[] args) {
DrawFn frame=new DrawFn();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
}

public DrawFn() {
super();
getC