Plotting expression using gplot

Hi,
I need some help regarding storing expression in plotlist. A command like
plot tran v(1)-v(2) will enlist v(1) and remove v(2) from plotlist.
What i want is to store v(1)-v(2) as a single probe. Is this possible?
I am searching for solution in u_prblst.cc but don't know whether its
a right place or not. And may be modifying something there will affect
other command too.

*You can checkout waveform displayer here.
http://git.savannah.gnu.org/cgit/gnucap.git/log/?h=AbhinavSingh-WIP

Though its not completed but its working fine as basic plot plugin.For
how to use it check here.
http://gnucap.org/dokuwiki/doku.php?id=gnucap:user:graphical_output_display_plugin

*Any suggestion or feedback will be helpful.

Thanks,
Abhinav