line() not drawing anything.

Hi. I am pretty new to processing so bear with me for silly mistakes.I am trying to draw a trace of data received on serial port. I am able to receive it but i am not able to draw in using line. It just seems to not respond. The code is following which is derived form code i found online

import processing.serial.*;

Serial myPort;  // Create object from Serial class
float val, screen_increment, old_x=0, old_y=0;      // Data received from the serial port
String inString;  // Input string from serial port


void setup() 
{

  size(1000, 600);
  String portName = Serial.list()[2];//Set the Serial port COM    
  myPort = new Serial(this, portName, 9600);//Set up the serial port
  myPort.bufferUntil('\n');//read in data until a line feed, so the arduino must do a println
  background(208,24,24);//make the background that cool blood red
}//setup

void draw()
{
//nothing in here, this is kind of like the void loop in arduino
}

void serialEvent(Serial myPort) { //this is called whenever data is sent over by the arduino

  inString = myPort.readString();//read in the new data, and store in inString
  inString = trim(inString);//get rid of any crap that isn't numbers, like the line feed
  val = float(inString);//convert the string into a number we can use

  val=map(val,0,4096,0,600);

  strokeWeight(5);//beef up our white line
  stroke(255, 255, 255);//make the line white

  //drawing the line
  println(old_x,old_y,screen_increment,600-val);
  line(old_x, old_y, screen_increment, 600-val);


  //store the current x, y as the old x,y, so it is used next time
  old_x = screen_increment;
  old_y = 600-val;

  //increment the x coordinate,  you can play with this value to speed things up
  screen_increment=screen_increment+2;

  //this is needed to reset things when the line crashes into the end of the screen
  if(screen_increment>(1000)){
    background(208,24,24); //refresh the screen, erases everything
    screen_increment=-50; //make the increment back to 0, 
    //but used 50, so it sweeps better into the screen
    //reset the old x,y values
  old_x = -50;
  old_y = 0;

  }

}

SInce i have added a println the output of the code println(old_x,old_y,screen_increment,600-val); is following which seems correct to me

    0.0 0.0 0.0 302.9297
    0.0 302.9297 2.0 303.8086
    2.0 303.8086 4.0 300.29297
    4.0 300.29297 6.0 297.36328
    6.0 297.36328 8.0 295.60547
    8.0 295.60547 10.0 294.87305
    10.0 294.87305 12.0 294.14062
    12.0 294.14062 14.0 294.87305
    14.0 294.87305 16.0 296.77734

All I see is blank red screen

Please Help