function setup() {

  createCanvas(400, 400);

  sx = width/2;

  sy = height/2;

  x = sx;

  y = sy;

}

​

function draw() {

  background(255);

  let slope = map(mouseY, 0, height, -10, 10);

  let speed = map(mouseX, 0, width, 0, 10);

  let xspeed = 1;

  let yspeed = xspeed *slope;  

  let rspeed = dist(0, 0, xspeed, yspeed);

  xspeed /= rspeed;

  yspeed /= rspeed;

  xspeed *= speed;

  yspeed *= speed;

  x += xspeed;

  y += yspeed;

  line(sx, sy, x, y);

  if(y > height || y < 0) y = height/2;

  if( x > width || x < 0) x = width/2;

}