Kind of like this: http://en.wikipedia.org/wiki/Lemniscate_of_Bernoulli

Lemniskate6.pde

void setup() {
  size(900, 900);
  smooth();

}

//start parameter
float t = 0;
//density of two lines
float density = 0.05;
//width of drawing
float a = 0.3;

void draw() {
  noStroke();
  fill(255, 0);
  rect(0, 0, width, height);

  t = t + density;
  a = a + density/100;
  
  float x = (a*sqrt(2)*cos(t))/(sq(sin(t))+1);
  float y = (a*sqrt(2)*cos(t)*sin(t))/(sq(sin(t))+1);
  
  float x2 = (a*sqrt(2)*cos(t+density))/(sq(sin(t+density))+1+0.2);
  float y2 = (a*sqrt(2)*cos(t+density)*sin(t+density))/(sq(sin(t+density))+1);
  
  strokeWeight(1);
  stroke(1);

  line(x*300+450,y*300+450,x2*300+450,y2*300+450);
}