code: def count_authors_by_annotation(graph: BELGraph, annotation: str = 'Subgraph') -> Mapping[str, typing.Counter[str]]:
      
      authors = group_as_dict(_iter_authors_by_annotation(graph, annotation=annotation))
      return count_defaultdict(authors)
docs: Group the author counters by sub-graphs induced by the annotation.
  
      :param graph: A BEL graph
      :param annotation: The annotation to use to group the graph
      :return: A dictionary of Counters {subgraph name: Counter from {author: frequency}}