/usr/lib/libplplotcxxd.so.11 is in libplplot-c++11 5.10.0-0ubuntu5.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/libplplotcxxd.so.11.0.0
/usr/lib/libplplotcxxd.so.11.0.0