/usr/lib/x86_64-linux-gnu/libplplot.so.15 is in libplplot15 5.13.0+dfsg-6ubuntu2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libplplot.so.15.0.0
/usr/lib/x86_64-linux-gnu/libplplot.so.15.0.0