I have been using gnuplot normally, as far as I remember (what could be before upgrading OS X). Anyway, I've suddenly got the following problem trying to run it.
dyld: Library not loaded: /opt/local/lib/libgd.2.dylib
Referenced from: /usr/local/bin/gnuplot
Reason: image not found
Trace/BPT trap: 5
So, I selfupdated MacPorts, upgraded the outdated ports, and nothing changed. Forced uninstall gnuplot and reinstalled, still nothing. Indeed, I went on and checked /opt/local/bin folder, and I saw that libgd.dylib and libgd.3.dylib are available, but definitely no "2" is present. Any idea on how can I solve it?
usually gnuplot from macports is installed in /opt/local/bin/gnuplot your version is something else and in the directory /usr/local/bin/gnuplot. If you want to use the macport version you need to change your PATH variable
To chack this run:
PATH=/opt/local/bin gnuplot
if that's the case, you need either to remove the executable /usr/local/bin/gnuplot or to change the PATH variable in your .bash_profile (or similar)
As you said, the gnuplot with macports is linked against the version 3 of gd library (/opt/local/lib/libgd.3.dylib). Probably your version worked until it was linked against the version 2.
If you really want to use your version, make a symlink
sudo ln -s /opt/local/lib/libgd.3.dylib /opt/local/lib/libgd.2.dylib