'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):

    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)]
