When doing 3-d graphing in Maxima, Maxima relies on gnuplot. When installing Maxima, however, it did not also install gnuplot. Therefore, I propose that gnuplot is listed as a dependency of Maxima, since Maxima assumes the program is there, and without it, it will not do 3-d graphing or plotting.
Fixed in >=sci-mathematics/maxima-5.9.1-r2