def plot_signal(self, start=None, end=None):
        """
        使用热力图画出买卖信号
        """
        start = self.account.start_date if start is None else start
        end = self.account.end_date if end is None else end
        _, ax = plt.subplots(figsize=(20, 18))
        sns.heatmap(
            self.account.trade.reset_index().drop(
                'account_cookie',
                axis=1
            ).set_index('datetime').loc[start:end],
            cmap="YlGnBu",
            linewidths=0.05,
            ax=ax
        )
        ax.set_title(
            'SIGNAL TABLE --ACCOUNT: {}'.format(self.account.account_cookie)
        )
        ax.set_xlabel('Code')
        ax.set_ylabel('DATETIME')
        return plt