function setup() {

  createCanvas(640, 240);

  randomCounts = [];

  for (let i = 0; i < 20; i++){

    randomCounts.push(0);

  }

}

​

function draw() {

  background(255);

  // pick a random number 

  let index = int(random(randomCounts.length));

  // increase the count

  randomCounts[index]++;

  stroke(0);

  fill(175);

  let w = width/randomCounts.length;

  for(let x = 0; x < randomCounts.length; x++){

    rect(x*w, height-randomCounts[x], w-1, randomCounts[x])

  }

}