Output dependency graph as one of the supported GraphViz output formats.

    :param dict tree: dependency graph
    :param string output_format: output format
    :returns: representation of tree in the specified output format
    :rtype: str or binary representation depending on the output format

def dump_graphviz(tree, output_format='dot'):
    """Output dependency graph as one of the supported GraphViz output formats.

    :param dict tree: dependency graph
    :param string output_format: output format
    :returns: representation of tree in the specified output format
    :rtype: str or binary representation depending on the output format

    """
    try:
        from graphviz import backend, Digraph
    except ImportError:
        print('graphviz is not available, but necessary for the output '
              'option. Please install it.', file=sys.stderr)
        sys.exit(1)

    if output_format not in backend.FORMATS:
        print('{0} is not a supported output format.'.format(output_format),
              file=sys.stderr)
        print('Supported formats are: {0}'.format(
            ', '.join(sorted(backend.FORMATS))), file=sys.stderr)
        sys.exit(1)

    graph = Digraph(format=output_format)
    for package, deps in tree.items():
        project_name = package.project_name
        label = '{0}\n{1}'.format(project_name, package.version)
        graph.node(project_name, label=label)
        for dep in deps:
            label = dep.version_spec
            if not label:
                label = 'any'
            graph.edge(project_name, dep.project_name, label=label)

    # Allow output of dot format, even if GraphViz isn't installed.
    if output_format == 'dot':
        return graph.source

    # As it's unknown if the selected output format is binary or not, try to
    # decode it as UTF8 and only print it out in binary if that's not possible.
    try:
        return graph.pipe().decode('utf-8')
    except UnicodeDecodeError:
        return graph.pipe()