This has some questionable methodology.

    image = Image.open(file)
    greyscale =  image.convert('L')
    stat = ImageStat.Stat(greyscale)
    avg_lightness = int(stat.mean[0])

But also:

> What I’ve graphed here is just the brightness of the window chrome, which isn’t really representative of the actual total screen brightness.

No kidding. Also, Mac OS keeps changing the chrome, and this isn’t obviously a very useful measurement.