code: def value_change(self, vcdId, value):
          '''append change from VCD file signal data series'''
          self.idcode2series[vcdId].append((self.now, value)f value_change(self, vcdId, value):
          '''append change from VCD file signal data series'''
          self.idcode2series[vcdId].append((self.now, value))
docs: append change from VCD file signal data series