/usr/lib/x86_64-linux-gnu/libplplotadad.so is in libplplot-ada1-dev 5.10.0+dfsg-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libplplotadad.so.1
/usr/lib/x86_64-linux-gnu/libplplotadad.so.1