def gen_graph(self, days):
        """
        根据历史数据绘制折线图并且保存
        """
        gas_list = self.get_histroy_gas(days)
        if gas_list:
            x = []
            y = []
            for index, gas in enumerate(gas_list):
                x.append(index+1)
                y.append(gas)
            # 计算变化百分比
            percentage_change = [None]  # 第一个点的变化百分比设为None
            for i in range(1, len(y)):
                change = ((y[i] - y[i - 1]) / y[i - 1]) * 100
                percentage_change.append(change)

            # 生成折线图
            plt.plot(x, y)
            plt.title(f'{len(gas_list)} days')
            plt.xlabel('day')
            plt.ylabel('GAS')

            # 在每个坐标点上显示Y轴的值和变化百分比
            for i in range(len(x)):
                if percentage_change[i] is not None:
                    plt.annotate(f'{y[i]} ({percentage_change[i]:.2f}%)', (x[i], y[i]), textcoords="offset points",
                                 xytext=(0, 10), ha='center')
                else:
                    plt.annotate(f'{y[i]}', (x[i], y[i]), textcoords="offset points", xytext=(0, 10), ha='center')
            # 设置为整型
            plt.xticks(x)
            # 保存折线图
            plt.savefig(self.file_name)
            plt.close()