'Adds value at timestamp.
Values must be added in order of increasing timestamp.
Args:
value: An observed value.
timestamp: The timestamp at which value was observed.
Raises:
RuntimeError: If timestamp is smaller than the previous timstamp.'

def Append(self, value, timestamp):

    timestamp = self._NormalizeTime(timestamp)
    if (self.data and (timestamp < self.data[(-1)][1])):
        raise RuntimeError('Next    timestamp    must    be    larger.')
    self.data.append([value, timestamp])
