Magnitude is the "length" of a vector. i.e. the hypotenuse between x and y
var center;
function setup() {
createCanvas(500, 350).parent('canvas-container');
center = createVector(width/2, height/2);
}
function draw() {
var magnitude,
mouse = createVector(mouseX, mouseY);
background(255);
mouse.sub(center);
// Plot magnitude
magnitude = mouse.mag();
fill(0);
rect(0 , height + 1 - magnitude, 10, magnitude);
rect(width - 11, height + 1 - magnitude, 10, magnitude);
translate(center.x, center.y);
line(0, 0, mouse.x, mouse.y);
}