Cell
The cell of an axis enclosing a coordinate: the lower and upper indices of the axis values around it, and the weight of the coordinate between them, 0 at lower and 1 at upper. A coordinate on a value, or outside the axis, has lower equal to upper and weight 0.
Throws
if lower is negative, upper is not lower or the next index, or weight is not in [0, 1] (or is not 0 when lower equals upper).