Extra information
-----------------

You can add extra information to object graphs, if you desire.

    >>> x = []
    >>> y = [x, [x], dict(x=x)]
    >>> import objgraph
    >>> objgraph.show_refs([y], extra_info=lambda x: hex(id(x)),
    ...                    filename='extra-info.png')
    Graph written to ....dot (5 nodes)
    Image generated as extra-info.png

.. figure:: extra-info.png
