let xspeed = 1;

​

function setup() {

  createCanvas(400, 400);

  x = 10;

  y = 50;

}

​

function draw() {

  background(220);

  let speed = map(mouseX, 0, width, 0, 0.5);

  let slope = map(mouseY, 0, height, 0, 2);

  // Calculate the ratio relationship between

  // xspeed and  yspeed

  let xspeed = 1;

  let yspeed = xspeed*slope;

  // Calculate the new speed

  let new_speed = sqrt(sq(xspeed) + sq(yspeed))

  // Use the pythagorean theorem to calculate 

  // xspeed and yspeed for a diagonal speed of 1 pixel 

  xspeed /= new_speed; 

  yspeed /= new_speed;

  // Multiple the speed to scale it out

  x += xspeed*speed;

  y += yspeed*speed;

  line(0, 0, x, y);

}