img = gimp.image_list()[0]
for i in range(0, 4000, 4000/10):
   pdb.gimp_rect_select(img, i, 0, img.width/10, img.height, 2, 0, 0)
   pdb.gimp_context_set_foreground((int(255 * i / 4000.0), 0, 0))
   pdb.gimp_edit_fill(img.layers[0], 0)