Keep track of the last # (however many you need) values in an array and use that array of values to draw a graph.

Code Example

1. int num = 50;
2. float[] arrayOfFloats = new float[num];
3.  
4. void setup() {
5.   size(800,800);
6.   smooth();
7. }
8.  
9. void draw() {
10.   background(255);
11.  
12.   // copy everything one value down
13.   for (int i=0; i<arrayOfFloats.length-1; i++) {
14.     arrayOfFloats[i] = arrayOfFloats[i+1];
15.   }
16.  
17.   // new incoming value
18.   float newValue = noise(frameCount*0.01)*width;
19.  
20.   // set last value to the new value
21.   arrayOfFloats[arrayOfFloats.length-1] = newValue;
22.  
23.   // display all values however you want
    
24.   for (int i=0; i<arrayOfFloats.length; i++) {
25.     noStroke();
26.     fill(255,0,0,255);
27.     ellipse(width*i/arrayOfFloats.length+width/num/2,height-arrayOfFloats[i],width/num,width/num);
28.   }
29. }

A little while ago I just wrote some fast code to visualize the data I was getting from Arduino. It's in no way perfect but might give you some ideas:

1. import processing.serial.*;
2. Serial Arduino;
3. int ioval;
4. ArrayList wave = new ArrayList();
5. int waveLength = 880;
6. int offset = wHeight / 2;
7. float prevX = 0, prevY = 0;
8. float multiplier = 7.5;
9. 
   void setup() 
10. {
11.   size(900,600);
12.   // sexy black BG
13.   background(0);
14.   Arduino = new Serial(this, Serial.list()[1],9600);
15.   Arduino.bufferUntil('\n');
16. }
17. 
    
18. void draw()
19. {
20.   while(Arduino.available() > 0) {
21.     ioval = Arduino.read();
22.     wave.add(ioval);
23.   }
24.   // background reset on frame
25.   background(0); 
      if(wave.size() >= waveLength) 
26.   {
27.     wave.remove(0); 
28.   }
29.  
    
30.   // add samples to canvas
31.   for(int i = 0; i < wave.size(); i++)
32.   {
33.     stroke(255);    
34.     if(prevX == 0 && prevY == 0)
35.     {
36.       prevX = i;
37.       prevY = ((Integer)wave.get(i));
38.     }
39.     line(prevX, prevY, i, ((Integer)wave.get(i)));
40.     prevX = i;
41.     prevY = ((Integer)wave.get(i)); 
42.   }

As you can see I'm using an ArrayList because it's easy to add and remove records from it. As you (probably) won't be removing any records in the middle of the arraylist and only at the beginning, it's just a quick way to store your samples.