The file src/nanoflann.h is a copy of the nanoflann C++ library
(https://github.com/jlblancoc/nanoflann), used for nearest-neighbour search
in edge_bundle_mingle().

nanoflann is distributed under the 2-clause BSD license:

  Copyright 2008-2009  Marius Muja (mariusm@cs.ubc.ca). All rights reserved.
  Copyright 2008-2009  David G. Lowe (lowe@cs.ubc.ca). All rights reserved.
  Copyright 2011-2026  Jose Luis Blanco (joseluisblancoc@gmail.com).
  All rights reserved.

The full license text is retained in the header of src/nanoflann.h.