PImage img = loadImage("robotmarko-g.jpg");
size(256, 256);
background(255);
loadPixels(); 
img.loadPixels();

int[] h = new int[256];

for (int y = 0; y < img.height; y++)
  for (int x = 0; x < img.width; x++) {
    int imgIndex = x + y * img.width;
    int r = int(brightness(img.pixels[imgIndex]));
    h[r]++;
  }

int hMax = 0;
for (int i = 0; i < 256; i++)
  if (h[i] > hMax)
    hMax  = h[i];

stroke(0);
for (int i = 0; i < 256; i++)
  line(i, height - 1, i, (height - 1) * (1 - (float) h[i] / hMax));