Plotting equations on a cartesian Plane

Hey, Im sort of a beginner of java, and i at the moment of working on a program that takes the input of an equation (eg. y=3x) and plots possible values for it on a cartesian plane thats on a seperate JPanel.

Here is my code: This is Class 1

import java.awt.Color;
import java.awt.Desktop;
import java.awt.Toolkit;
import java.awt.datatransfer.Clipboard;
import java.awt.datatransfer.StringSelection;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.io.File;
import java.io.IOException;

import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;

public class Graphs extends JFrame implements KeyListener
{
    GraphingPanel p = new GraphingPanel();
    JPanel[] Panel = new JPanel[2];
    JLabel[] Label = new JLabel[100];
    JTextField[] Field = new JTextField[100];
    JButton[] Enter = new JButton[100];
    JButton[] Clear = new JButton[100];
    JComboBox Subjects = new JComboBox();
    JComboBox Topics = new JComboBox();

    public Graphs()
    {
        super();
        setSize(600, 500);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setUndecorated(true);
        setLocationRelativeTo(null);
        LoadUI();
    }

    public void LoadUI()
    {
        Field[0] = new JTextField();
        Panel[0] = new JPanel(null);
        Panel[1] = p;


        Field[0].setBounds(240, 20, 120, 30);
        Field[0].addKeyListener(this);

        Panel[0].add(Field[0]);
        Panel[0].add(Panel[1]);
        add(Panel[0]);
        setVisible(true);
    }

    public static void main(String[] args)
    {
        Graphs Main = new Graphs();
    }

    @Override
    public void keyPressed(KeyEvent e)
    {
        if(e.getKeyCode()==10)
        {
            String str = Field[0].getText();
            DrawGraph g = new DrawGraph(str);
        }
    }

    @Override
    public void keyReleased(KeyEvent arg0) {
        // TODO Auto-generated method stub

    }

    @Override
    public void keyTyped(KeyEvent arg0) {
        // TODO Auto-generated method stub

    }
}

The Second Class:

import java.awt.*;
import java.awt.geom.*;

import javax.swing.*;

public class GraphingPanel extends JPanel
{
    public GraphingPanel()
    {
        setBounds(50, 50, 500, 400);
        setBorder(BorderFactory.createLineBorder(Color.BLACK, 2));
        setBackground(Color.WHITE);
    }

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

        int panelWidth = getWidth();
        int panelHeight = getHeight();

        g.setColor(Color.WHITE);
        g.fillRect(0, 0, panelWidth, panelHeight);
        g.setColor(Color.BLACK);

        Graphics2D g2 = (Graphics2D) g;
        g2.setStroke(new BasicStroke(1f));


        g2.draw(new Line2D.Double( 0, panelHeight/2, panelWidth ,panelHeight/2 ));
        g2.draw(new Line2D.Double(panelWidth/2,0, panelWidth/2 ,panelHeight));
        g2.setFont(new Font("Times New Roman", Font.PLAIN, 13));

        for(int i=1;i<getWidth();i+=9)
        {
            g2.draw(new Line2D.Double(i, (getHeight()/2)-2, i, (getHeight()/2)+2));
        }

        for(int i=1;i<getHeight();i+=9)
        {
            g2.draw(new Line2D.Double((getWidth()/2)-2, i,(getWidth()/2)+2,i));
        }
    }
}

Im just struggling to figure out how to plot a line graph after the user has input his equation into the Text field.. Your help will be greatly appreciated:)