This example was uploaded to Youtube last year.

It is based in the code at

The Processing code was modified as is explained in the comments.

// Pro_Graph2.pde
 Used in the Youtube video "Arduino and Processing ( Graph Example )"
 Based in the Tom Igoe example.
 Mofified by Arduining 17 Mar 2012:
  -A wider line was used. strokeWeight(4);
  -Continuous line instead of vertical lines.
  -Bigger Window size 600x400.
This program takes ASCII-encoded strings
from the serial port at 9600 baud and graphs them. It expects values in the
range 0 to 1023, followed by a newline, or newline and carriage return

Created 20 Apr 2005
Updated 18 Jan 2008
by Tom Igoe
This example code is in the public domain.

import processing.serial.*;

Serial myPort;        // The serial port
int xPos = 1;         // horizontal position of the graph 

//Variables to draw a continuous line.
int lastxPos=1;
int lastheight=0;

void setup () {
  // set the window size:
  size(600, 400);        

  // List all the available serial ports
  // Check the listed serial ports in your machine
  // and use the correct index number in Serial.list()[].

  myPort = new Serial(this, Serial.list()[0], 9600);  //

  // A serialEvent() is generated when a newline character is received :
  background(0);      // set inital background:
void draw () {
  // everything happens in the serialEvent()

void serialEvent (Serial myPort) {
  // get the ASCII string:
  String inString = myPort.readStringUntil('\n');
  if (inString != null) {
    inString = trim(inString);                // trim off whitespaces.
    float inByte = float(inString);           // convert to a number.
    inByte = map(inByte, 0, 1023, 0, height); //map to the screen height.

    //Drawing a line from Last inByte to the new one.
    stroke(127,34,255);     //stroke color
    strokeWeight(4);        //stroke wider
    line(lastxPos, lastheight, xPos, height - inByte); 
    lastxPos= xPos;
    lastheight= int(height-inByte);

    // at the edge of the window, go back to the beginning:
    if (xPos >= width) {
      xPos = 0;
      lastxPos= 0;
      background(0);  //Clear the screen.
    else {
      // increment the horizontal position:

Important Note:
Remember to replace the index in Serial.list()[0] to open the Serial Port where the Arduino is connected.
Run the program in Processing and check the Console window to see the available ports.