I had the same problem. After lurking a while in bugzilla, I found the solution!

You said you reflashed your N900, like me. So it is very probable some files were still on your /opt/(something) directory. Search for a gnuplot directory in there and remove. (But do not use #rm -rfr gnu*, because there is a folder called gnu which is totally innocent!).

After that, get aptitude, and enter in terminal:

#sudo gainroot
#aptitude install gnuplot*

Aptitude gives you a much more complete feedback. It may seem that the installation failed again, but when you open gnuplot, you will see "Terminal type set to x11", instead of the old "Terminal type set to unknown". This means you now can plot your graphics as you should do :-)

Hope this helps.