void setup(){
  size(300,300);
  frameRate(60);
}
float x=150,y=150,xn=150,yn=150;
void draw(){
  //background(255);

  if(random(1.)>0.5)
    x = x + random(-10,10);
  else
    y = y + random(-10,10);
  x = round(x/10.)*10; //snap
  y = round(y/10.)*10;
  x = constrain(x,0,width);
  y = constrain(y,0,height);
  line(x,y,xn,yn);
  xn = x;
  yn = y;
}