plotutils: add license (#74566)
plotutils: add license (#74566)

The COPYRIGHT file is the GPLv3 template, and most of the source headers
with a license annotation indicate that any later version of the license
can be used.
JCount/homebrew-core,lemzwerg/homebrew-core,uyjulian/homebrew-core,nandub/homebrew-core,mfikes/homebrew-core,sjackman/homebrew-core,cblecker/homebrew-core,makigumo/homebrew-core,adam-moss/homebrew-core,filcab/homebrew-core,mbcoguno/homebrew-core,zyedidia/homebrew-core,Homebrew/homebrew-core,filcab/homebrew-core,Homebrew/homebrew-core,zyedidia/homebrew-core,Moisan/homebrew-core,adam-moss/homebrew-core,stgraber/homebrew-core,lemzwerg/homebrew-core,mbcoguno/homebrew-core,Linuxbrew/homebrew-core,sjackman/homebrew-core,nbari/homebrew-core,chrisfinazzo/homebrew-core,cblecker/homebrew-core,nandub/homebrew-core,JCount/homebrew-core,mbcoguno/homebrew-core,Moisan/homebrew-core,Linuxbrew/homebrew-core,chrisfinazzo/homebrew-core,makigumo/homebrew-core,stgraber/homebrew-core,mfikes/homebrew-core,nbari/homebrew-core,uyjulian/homebrew-core