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 <ssuominen@gentoo.org> + +libqalculate-0.9.7-r1.ebuild: + Optionalize sci-visualization/gnuplot with USE="gnuplot" wrt #425622 by Maxim + Kammerer (and workaround hardcoded HTML reference directory in install phase)