int[] randomCounts;

void setup() {
  size(640, 240);
  randomCounts = new int[20];
}

void draw() {
  background(255);
  int index = int(random(randomCounts.length));
  randomCounts[index]++;
  stroke(0);
  fill(175);
  int w = width/randomCounts.length;
  
  for(int x = 0; x < randomCounts.length; x++) {
    rect(x * w, height - randomCounts[x], w - 1, randomCounts[x]);
  }
}