Graphviz

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Skip empty ``comment`` when creating DOT source.

Document ``graph_attr``, ``node_attr``, and ``edge_attr`` in the README.

More informative exception when Graphviz executables cannot be called.

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Fixed missing identifier quoting for DOT keywords (thanks to Paulo Urio).

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Made ``format`` and ``engine`` case-insensitive.

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Indent ``graph_attr``, ``node_attr``, and ``edge_attr`` lines, adapt nodes and edges.

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Fixed ``view()`` failing on paths with forward slashes on Windows.

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Added Python 3.3+ support.

Made attributes order stable (sorting plain dicts).

Fixed edgeop in undirected graphs.