code: def id_offset(cls):
          
          max_ids = []
          for backend in Store.renderers.keys():
              store_ids = Store.custom_options(backend=backend).keys()
              max_id = max(store_ids)+1 if len(store_ids) > 0 else 0
              max_ids.append(max_id)
          # If no backends defined (e.g plotting not imported) return zero
          return max(max_ids) if len(max_ids) else 0
docs: Compute an appropriate offset for future id values given the set
          of ids currently defined across backends.