The AL-Spline-Matrix spreadsheet contains a large number of VBA user defined functions (UDFs) performing spline fitting and matrix operations, using the free and open source ALGLIB library. It was recently pointed out that the Excel interface code for the CSplineFitCW1da function had an error. The function fits a cubic spline to scattered data, with provision for weighting and specified constraints, but the code error resulted in the constraints not working properly. The code has now been fixed, and an example with constrained data is shown below.
The revised spreadsheet, including full open source code, (Version 1.33; 27 Nov 2017) can be downloaded from:
AL-Spline-Matrix03.zip (Excel 2003 version)
AL-Spline-Matrix07.zip (Excel 2007 and later)
The screen-shot below shows results with constraints applied at the minimum and maximum x-values, compared with unconstrained results, and the Hermite Fit function: