/usr/lib/graphviz/libgvplugin_gtk.so.6 is in libgvc6-plugins-gtk 2.36.0-0ubuntu3.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/graphviz/libgvplugin_gtk.so.6.0.0
/usr/lib/graphviz/libgvplugin_gtk.so.6.0.0