right, if the points are distributed in a square then the distribution looks more like what one would expect:

(i am to lazy to register to make a fork, here is the code change):

replace:

  let a = random(Math.PI*2);
  let r = d * (1 - random());
  point(cos(a)*r, sin(a)*r);

with:

  let x = random(d);
  let y = d * (1 - random());
  point(x, y);

in every block

you could even simplify it to:

  let x = random(d);
  let y = random(d);
  point(x, y);

and for more fun apply the nested random() call to both axis:

  let x = random(random(d));
  let y = random(random(d));
  point(x, y);