function setup() {
  createCanvas(400, 400);
  background(220);
  var step = 400 / 9;

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

  for (var i = 0; i < 2; i++) {
    line(...line_xys[i]);
  }

  for (var i = 0; i < 9; i++) {
    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);
  }
}