changes:
- arguments are passed to histogram2d in the same order,
  i.e., one has to specify histogram2d_min_max(y, x)
  for relating the first index to y and the second to x
  (as assumed by most functions like plot_image, png_write, etc.)
- xmin, xmax, ymin, ymax qualifiers
- last bin does not start at {x/y}max, but stop there