This example shows how to read serial data from your Arduino into Processing.

Arduino setup

Connect three photoresistors via a 10K resistor to a 5V power source (you can use your Arduino).
Connect the analog inputs A0, A1 and A2 as in the image below:

Arduino sketch

void setup() { 
  Serial.begin(9600);	
}

void loop() { 
  Serial.write(map(analogRead(A0),0,1023,1,66)); 
  delay(50); 
  Serial.write(map(analogRead(A1),0,1023,67,132)); 
  delay(50); 
  Serial.write(map(analogRead(A2),0,1023,133,199)); 
  delay(50); 
}

Processing Code

/*
Arduino - Read and graph multiple values 
          from Serial Port example
*/

import processing.serial.*;

Serial myPort;

int mWidth=1000;
int mHeight=200; 
int mValue;
int[][] mPoints;

void setup() {

  // set window size and background color 
  size(mWidth, mHeight);

  // init objects/vars
// make sure to replace COM3 with the correct port name:
  myPort = new Serial(this, "COM3", 9600);
  mPoints = new int[mWidth][2];
  for(int i=0;i<Width;i++){ mPoints[i][0]=i; mPoints[i][1]=mHeight; } } void draw() { if ( myPort.available() > 0) {
    background(0); // removes all prior drawn lines

    // draw dividers
    stroke(0,0,120); // dark blue
    line(0,67,mWidth,67);
    line(0,133,mWidth,133);
    stroke(0,255,0); // back to green

    // read the value
    mValue = myPort.read();

    for(int i=0;i<mWidth;i++){ if(i==mWidth-1){ mPoints[i][1] = mHeight-mValue; }else{ mPoints[i][1] = mPoints[i+1][1]; } if(i>3){
        line(mPoints[i-3][0],mPoints[i-3][1],mPoints[i][0],mPoints[i][1]);

      }

    } // next for
  } // end if port available
}

The result is a real-time graph that shows the light intensity: