/usr/lib/x86_64-linux-gnu/libplotter.so is in libplot-dev 2.6-9.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libplotter.so.2.2.4
/usr/lib/x86_64-linux-gnu/libplotter.so.2.2.4