immutable x values for interpolant
f(x) values for interpolant
SplineBoundaryCondition for left tail.
SplineBoundaryCondition for right tail.
SplineType type of cubic spline.
tangent power parameter for cardinal SplineType (ignored by other spline types). Use 1 for zero derivatives at knots and 0 for Catmull–Rom spline. Constraints: grid and values must have the same length >= 3