/usr/lib/tcltk/graphviz/libtkspline.so is in libgv-tcl 2.38.0-12ubuntu2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/tcltk/graphviz/libtkspline.so.0.0.0
/usr/lib/tcltk/graphviz/libtkspline.so.0.0.0