void setup() {
  size(200, 200);
  background(255);
}

void draw() {
  background(255);
  
  // three upper lines
  line(10, 10, mouseX, mouseY);
  line(100, 10, mouseX, mouseY);
  line(190, 10, mouseX, mouseY);
  
  // two middle lines
  line(10, 100, mouseX, mouseY);
  line(190, 100, mouseX, mouseY);
  
  // three bottom lines
  line(10, 190, mouseX, mouseY);
  line(100, 190, mouseX, mouseY);
  line(190, 190, mouseX, mouseY);
}