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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def FetchMostRecentGraphSeries(label, report_type, token = None ):
 """Fetches the latest graph series for a client label from the DB. Args: label: Client label to fetch data for. report_type: rdf_stats.ClientGraphSeries.ReportType to fetch data for. token: ACL token to use for reading from the legacy (non-relational) datastore. Raises: AFF4AttributeTypeError: If, when reading to the legacy DB, an unexpected report-data type is encountered. Returns: The graph series for the given label and report type that was last written to the DB, or None if no series for that label and report-type exist. """

  if _ShouldUseLegacyDatastore():
    return _FetchMostRecentGraphSeriesFromTheLegacyDB(
        label, report_type, token=token)

  return data_store.REL_DB.ReadMostRecentClientGraphSeries(label, report_type)