p5jsでグラフを描く

function setup() {
  createCanvas(400, 400);
  console.log(width/2);
}

function draw() {
  background(220);
  translate(width/2,height/2);
  scale(1,-1);
  stroke(128);
  line(-width/2, 0, width/2, 0);
  line(0, -height/2, 0, height/2);
  
  scl = 20;
  
  x1 = -width/2;
  x2 = width/2;
  
  y1 = (2 * scl) * x1 / scl -(1 * scl);
  y2 = (2 * scl) * x2 / scl -(1 * scl);
  
  stroke(255,0,0);
  line(x1, y1, x2, y2);
 

  y1 = -(1/2 * scl) * x1 / scl + (13/2 * scl);
  y2 = -(1/2 * scl) * x2 / scl + (13/2 * scl);
  
  stroke(0,0, 255);
  line(x1, y1, x2, y2);
}