Convert the sequence to the sequence of differences between points.

    The value of each point v[i] is replaced by v[i+1] - v[i], except for the
    last point which is dropped.

def ToDeltas(self):
    """Convert the sequence to the sequence of differences between points.

    The value of each point v[i] is replaced by v[i+1] - v[i], except for the
    last point which is dropped.
    """
    if len(self.data) < 2:
      self.data = []
      return
    for i in range(0, len(self.data) - 1):
      if self.data[i][0] is None or self.data[i + 1][0] is None:
        self.data[i][0] = None
      else:
        self.data[i][0] = self.data[i + 1][0] - self.data[i][0]
    del self.data[-1]