float x, y, px, py;
float theta;
float rotation;

void setup() {
  size(300, 300);
  x = y = px = py = theta = 0;
  rotation = 0.1;
  background(255);
}

void draw() {
  // find coordinates with rotating reference frame
  pushMatrix();
  rotate(theta/PI);
  x = screenX(theta, 0);
  y = screenY(theta, 0);
  popMatrix();

  translate(width/2.0, height/2.0);
  theta += rotation;
  line(px, py, x, y);
  px = x;
  py = y;
  if (theta>width/2.0) frameCount=-1; // start over
}