This comment about "causes an error" (a) isn't true, and (b) shouldn't be true.  It is also the interpolant used for the internal LookupTable.

This raises the question as to whether this should be the case.  Perhaps the internal LookupTable should always be linear to assure monotonicity of the interpolation.  Then the documented behavior would be appropriate -- the user would be declaring how it wants the input file to be interpolated, so an error should then be raised when it is passed without using the file option.