sci-visualization/gnuplot is a completely optional dependency of sci-libs/libqalculate -- gnuplot will be used if found, but is not required for correct operation. Since gnuplot is a heavy dependency, I suggest either splitting it into a USE flag, or notifying the user about the possibility to install gnuplot pkg_postinst().
Propably shouldn't have needed a revision bump, but being extra safe...
+*libqalculate-0.9.7-r1 (10 Jul 2012)
+ 10 Jul 2012; Samuli Suominen <firstname.lastname@example.org>
+ Optionalize sci-visualization/gnuplot with USE="gnuplot" wrt #425622 by Maxim
+ Kammerer (and workaround hardcoded HTML reference directory in install phase)