def _get_graph_title(self):
        """获取图像的title."""
        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