docs: 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,
          )