/usr/lib/x86_64-linux-gnu/libplplottcltk.so.13 is in plplot-tcl 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/libplplottcltk.so.13.0.1
/usr/lib/x86_64-linux-gnu/libplplottcltk.so.13.0.1