Highlighting
------------

You can highlight some graph nodes.

    >>> class Node(object):
    ...     def __init__(self, *neighbours):
    ...         self.neighbours = list(neighbours)
    >>> a = Node()
    >>> b = Node(a)
    >>> c = Node(b)
    >>> d = Node(c)
    >>> a.neighbours.append(d)

    >>> import objgraph
    >>> objgraph.show_backrefs(a, max_depth=15,
    ...     extra_ignore=[id(locals())],
    ...     highlight=lambda x: isinstance(x, Node),
    ...     filename='highlight.png') # doctest: +NODES_VARY
    Graph written to ....dot (12 nodes)
    Image generated as highlight.png

.. figure:: highlight.png
   :scale: 50%

