Shorter version of some code I found online (can't trace back where). I think it moves you in the right direction.

1. ArrayList hist = new ArrayList();
2. float joinDist = 90;
3. 
   
4. void setup() {
5.   size(800,600);
6.   background(255);
7. }
8. 
   
9. void draw() {
10. }
11. 
    
12. void mouseDragged() {
13.   stroke(0, 25);
14.   PVector d = new PVector(mouseX, mouseY, 0);
15.   hist.add(0,d);
16.   for (int p = 0; p < hist.size(); p++) {
17.     PVector v = (PVector) hist.get(p);
18.     float joinChance = p/hist.size() +
19.       d.dist(v)/joinDist;
20.     if (joinChance < random(0.4))
21.       line(d.x, d.y, v.x, v.y);
22.   }
23. }
24. 
    
25. void keyPressed() {
26.   if (key == ' ') {
27.     background(255);
28.     hist.clear();
29.   }
30. }

here is a more complex version of it from the book "generative Gestaltung"
http://www.generative-gestaltung.de/M_2_6_01_TOOL