Coordinates are normalized on maximum possible luminosity value, i.e. you specify values in range [0, 1] and they are stretched either to [0, 255] or [0, 65535] depending on extended property value. Note, you still can specify values that are out of [0, 1] range, and algorithm automatically truncates the curve. If all points will be strictly inside of this range, algorithm will add points at:
x equal to 0 and y equal to Y-coordinate of the leftmost point.
x equal to 1 and y equal to Y-coordinate of the rightmost point.