code: def plot_signal(self, start=None, end=None):
          
          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
docs: 使用热力图画出买卖信号