Getting a sensor to create an image in Processing:

In Arduino write this code (for a sensor on pin 0):

int sensorPin = 0;

void setup() 
{
 Serial.begin(9600);
 pinMode(sensorPin, INPUT); //my sensor will send data in to the Arduino
}

void loop()
{
 int volt = analogRead(sensorPin);  //read the data
 //Serial.println(num);  //find out what the number range is
 int num = map(volt, 0, 75, 0, 800);  
//remap the numbers so they result in ones you can use

 Serial.write(num);  //write the sensor data so Processing can capture it
 delay(100);
}

In Processing:

import processing.serial.*;

Serial myPort; // Create object from Serial class
String val; // used for data received from the serial port
float Pos = 0; //used in drawing

void setup() {
  String portName = Serial.list()[2]; 
  //change the 0 to a 1 or 2 etc. to match your port
  myPort = new Serial(this, portName, 9600);

  size(800, 600); //create a window to draw in
  background(0); //set the background color (black)
}

void serialEvent (Serial myPort) {
   //a new function that looks for Serial events
  int sensorData = myPort.read(); // get the sensor data
  //println(sensorData); // print it so we can see it
  Pos = sensorData; //use the sensor data to draw a line (later)
}

void draw() {
  line(0, Pos, 800, Pos); //draw a line x1, y1, x2, y2
  stroke(random(255), 0, Pos); //change the color of the stroke randomly
  line(Pos, 0, Pos*3, 600);
  stroke(Pos, 0, Pos);
}

Diagram:

Resources

Processing Info on Line()

The attributes that work with Line are:

strokeWeight()
strokeJoin()
strokeCap()
beginShape()