@Suiseiseki Your point is completely true and that's what i wanted to say too.
However, the fact that we can't make derivation of Gnuplot is kinda crucial defect when one assumes the program is a part of the GNU project but actually it's not. Because the freedom to make a derivation of the program is one of essential four freedoms that is advocated by many free software activists. At least, every program of GNU fulfills those 4 freedoms.