"My intension behind is, to avoid screenshots in this forum that contains either no TX-RX distance calculations or wrong distances. The distance and direction from TX is quite important, especially for broadcasters."
I think there is only a very low percentage of beginner's reports with missing distances, and you still depend on the user to use the right coordinates. So a correct distance is still not sure.
"I changed the background color, because there were some complaints about the legibility of the yellow graph on some XP desktops schemes. Now there is a maximum legibility on all desktop schemes and this is also important."
Maybe for programmes and machines, I cannot read the scaling lines anymore.
"Everybody has a different taste and that is the reason why I will not make colors user changeable. Then everybody would have his own colors and it would be quite difficult to compare the results."
For me this all is not a matter of taste but - as you want- a matter of legibility. Why not use a white background and something different than yellow then. I don't care if it's white or grey background, but black is the worst solution for my eyes.
By the way, I just tested the printing option with my Epson printer.
Do you have a contract with the ink industry?
My printer really prints the black background, the paper is all wet!
Again I think white background would be the best solution.