Original pic was written to go with Joseph Ossanna’s original troff(1) by Brian Kernighan, and later re-written by Kernighan with substantial enhancements (apparently as part of the evolution of troff(1) into ditroff(1) to generate device-independent output).
The language had been inspired by some earlier graphics languages including ideal and grap. Kernighan credits Chris van Wyk (the designer of ideal) with many of the ideas that went into pic.
The pic language was originally described by Brian Kernighan in Bell Labs Computing Science Technical Report #116 (you can obtain a PostScript copy of the revised version, [1], by sending a mail message to netlib@research.att.com with a body of ‘send 116 from research/cstr’). There have been two revisions, in 1984 and 1991.
The document you are reading effectively subsumes Kernighan’s description; it was written to fill in lacunæ in the exposition and integrate in descriptions of the GNU gpic(1) and pic2plot(1) features.
The GNU gpic implementation was written by James Clark ⟨ jjc@jclark.com⟩ .
The GNU pic2plot implementation is based on James Clark’s parser code and maintained by Robert Maier, principal author of plotutils.