void setup(){
  size(300, 200);
  smooth();
}

void draw(){
  background(255);
  stroke(200);
  strokeWeight(5);
  for(int i=0; i<300; i+=10){
  line(i, 0, i*3, height);
  }
  if(mousePressed){
    strokeWeight(10);
    stroke(10, 150, 100);
  }else{
    stroke(200, 0, 0);
  }
  line(50, 0, 150, height);
}