The Excel Spline-Matrix spreadsheet has now been updated with several new spline interpolation functions. See Alglib/Python linear and non-linear fitting functions for links to download and install the necessary Excel-Python and Alglib files. Also required is Python version 2.7 or 2.6.
Of which, the new ones are:
The parametric spline function generates a smooth curve through a series of points, which need not be in order of increasing X. Output may be specified to be periodic, in which case the curve is extended to the starting point, with slopes adjusted to provide a smooth curve throughout:
xl_Ratinterp provides rational interpolation without poles. See the Alglib User Guide for background information.