Uneven spacing of points

Hi

I'm plotting integer values with points and use ps 0. The resulting PNG
(example: http://tinyurl.com/59glml)
has uneven vertical spacing of the
points, which  gives the visual impression of thin white horizontal bands.
I guess that this is a more generic image rendering problem rather than a
gnuplot issue and one can't do very much, apart from increasing the point
size (which I want to avoid).

Or is there any other workaround?

Thanks in advance, Hermann