Gnuplot included unclear/unknown licensed code.
#File List Start:
This file (B) copyright infomation is:
"Copyright (c) 1992 Robert K. Cunningham, MIT Lincoln Laboratory"
#File List End:
A) "Open Source" is not license.
B) No license infomation. but This author is listed GNUPlot Contributor. See: http://www.gnuplot.info/docs_5.0/gnuplot.pdf
This problem already reported. (Gnewsense).
I suggest that resolve license problem.
It is highly likely that Petr Mikulik, as a listed "substantial contributor" to gnuplot (http://www.gnuplot.info/faq/faq.html) considers his code in gnuplot to be under the gnuplot license, despite the poor attribution in color.c. The same is true for Robert Cunningham.
Given that these files are distributed in a tarball with a license that applies to the entire work (gnuplot license) with these contributors being openly recognized for almost 20 years, I think it is safe to treat these files as being under the terms of the gnuplot license.