Compute an appropriate offset for future id values given the set
        of ids currently defined across backends.

def id_offset(cls):
        """
        Compute an appropriate offset for future id values given the set
        of ids currently defined across backends.
        """
        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