It looks like gnuplot depends on two other packages, gnuplot-x11 and gnuplot-nox, but only gnuplot-x11 is being fetched here. That could be because gnuplot-nox is already there for some reason.

Anyway try removing all gnuplot packages from your system and then reinstalling it.

Become root:
$ sudo gainroot

Uninstall all gnuplot packages:
# apt-get purge gnuplot*

Reinstall gnuplot:
# apt-get install gnuplot

Also make sure you have the testing repository enabled but not the devel repository (just in case there are packages in both).

Best of luck