code: def per_system_data(self):
          
          ret = {}
          for cache_date in self.cache_dates:
              data = self._cache_get(cache_date)
              ret[cache_date] = {
                  self._column_value(x): data['by_system'][x]
                  for x in data['by_system']
              }
              if len(ret[cache_date]) == 0:
                  ret[cache_date]['unknown'] = 0
          return ret
docs: Return download data by system.
  
          :return: dict of cache data; keys are datetime objects, values are
            dict of system (str) to count (int)
          :rtype: dict