The implementation we have in illumos-gate of libplot is horribly, horribly bitrotted. To the point where its both highly unlikely its used by anything, and probably very difficult for anyone to actually use productively.
Its time to remove these last bits.
Distributions should provide a working GNU plotutils library for applications that need it. (The GNU plotutils stuff is much newer, and largely API compatible.)
Much of this is covered by PSARC cases 2009/540 and 2009/585.