docs: def _add_signal_history(self, df, symbol):
          """ Initilize signal history """
          if symbol not in self.signals.keys() or len(self.signals[symbol]) == 0:
              self.signals[symbol] = [nan] * len(df.index)
          else:
              self.signals[symbol].append(nan)
  
          self.signals[symbol] = self.signals[symbol][-len(df.index):]
          signal_count = len(self.signals[symbol])
          df.loc[-signal_count:, 'signal'] = self.signals[symbol][-signal_count:]
  
          return df