Note that the GPL is incompatible with licenses that mandate distribution of forks as a patch or series of patches (like the gnuplot license).No, the mere fact of requiring that modified sources be distributed in base+patch form rather than as a monolithic tarball is not what makes the gnuplot licence incompatible with the GPL.
If you are shipping GPL-based code and, like the gnuplot authors, you want to insist on that behaviour, all you need to do with is add a clarifying note stating explicitly what we all know, which is that "where you distribute a modified form of the software, the preferred form for modifications is the original upstream code base, plus your patches to it."
In the gnuplot case, it is other clauses which impose additional restrictions that are incompatible with the GPL. For example the requirement to distribute the patches with the binaries, while the GPL only requires you to provide sources on demand. And the requirement to provide your name and address when you ship a modified version, etc.
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