code: def _get_graph_title(self):
          
          start_time = datetime.fromtimestamp(int(self.timestamp_list[0]))
          end_time = datetime.fromtimestamp(int(self.timestamp_list[-1]))
          end_time = end_time.strftime('%H:%M:%S')
          title = "Timespan: %s —— %s" % (start_time, end_time)
  
          return title
docs: 获取图像的title.