There is a beautiful script producing a rectangle with rounded corners here:
http://gnuplot-surprising.blogspot.de/2011/10/round-corner-rectangle-in-gnuplot.html
It works so far and I'm able to adapt this example to my needs.
The problem is that it does not work properly in case of pdf output and I have no idea why .. there seems to be a problem with pdf and filledcurve in a way, that the area is not properly filled. Viewing the pdf you can clearly see that the filling consists of stripes and in between them are white lines separating them. It looks really bad on screen and on paper.
Again: the problem only occurs if the script is adapted to pdf output!
I've been working on this all day and still no clue how to fix this.
Does anyone see a way out? Or is it an inherent problem of the way curves are filled?