docs: def vertical_white(width=10, height=100, subtlety=0.1):
      """
      Draws a subtle vertical gradient strip: white with varying alpha.
      """
  
      start = 0.5 - subtlety
      end = 0.5 + subtlety
  
      surface = cairo.ImageSurface(cairo.FORMAT_ARGB32, width, height)
      ctx = cairo.Context(surface)
  
      ctx.scale(width/1.0, height/1.0)
  
      pat = cairo.LinearGradient(0.0, 0.0, 0.0, 1.0)
      pat.add_color_stop_rgba(0, 1, 1, 1, start)
      pat.add_color_stop_rgba(1, 1, 1, 1, end)
  
      ctx.rectangle(0, 0, 1, 1)
      ctx.set_source(pat)
      ctx.fill()
  
      surface.write_to_png('vertical_white.png')