let num_of_line = 0;

function keyPressed() {
  if (keyCode === UP_ARROW) {
    console.log("up_arrow");
    num_of_line = num_of_line + 1;
  } else if (keyCode === DOWN_ARROW) {
    console.log("down_arrow");
    num_of_line = num_of_line - 1;
  }

  if (num_of_line < 0) num_of_line = 0;
  if (num_of_line > 9) num_of_line = 9;
}

function setup() {
  createCanvas(400, 400);
  background(220);
}

function draw() {
  background(220);

  var line_xys = [
    [0, 0, 400, 400],
    [400, 0, 0, 400],
  ];

  if (num_of_line > 0) line(...line_xys[0]);
  if (num_of_line > 1) line(...line_xys[1]);

  if (num_of_line > 2) {
    var need_to_draw = num_of_line - 2;
    for (var i = 1; i <= need_to_draw; i++) {
      var step = 400 / (need_to_draw + 1);
      var top_left = 0;
      var bottom_right = 400;
      var top_x = top_left + step * i;
      var bottom_x = bottom_right - step * i;
      line(top_x, 0, bottom_x, 400);
      console.log({ step, top_x, bottom_x });
    }
  }
}