Export graph as an image.

        This requires that Graphviz is installed and that the ``dot``
        executable is in your path.

        The *filename* argument specifies the output filename.

        The *format* argument lets you specify the output format.  It may be
        any format that ``dot`` understands, including extended format
        specifications like ``png:cairo``.  If omitted, the filename extension
        will be used; if no filename extension is present, ``png`` will be
        used.

        The *dot_executable* argument lets you provide a full path to the
        ``dot`` executable if necessary.

def export_image(self, filename='refcycle.png', format=None,
                     dot_executable='dot'):
        """
        Export graph as an image.

        This requires that Graphviz is installed and that the ``dot``
        executable is in your path.

        The *filename* argument specifies the output filename.

        The *format* argument lets you specify the output format.  It may be
        any format that ``dot`` understands, including extended format
        specifications like ``png:cairo``.  If omitted, the filename extension
        will be used; if no filename extension is present, ``png`` will be
        used.

        The *dot_executable* argument lets you provide a full path to the
        ``dot`` executable if necessary.

        """
        return self.annotated().export_image(
            filename=filename,
            format=format,
            dot_executable=dot_executable,
        )