# Spline interpolation function for Excel.

## Abstract

Have you ever wondered what smoothing algorithm Excel uses to fit smooth curves on a XY scatter? Have you ever found yourself trying to read Y values off an Excel XY scatter plot? Did you ever wish there was a simple way to linearly interpolate in Excel? If you’ve answered Yes to any of the questions above, you may find this new Excel add-in useful. What does this add-in do? It provides you with 2 new custom Excel functions: 1. A function to interpolate along Excel’s smooth line fit. This the ‘Bezier’ function. 2. A function to linearly interpolate and extrapolate. This is the ‘Linerp’ function. How does Excel compute its smoothing algorithm for curved lines in the XY scatter plots? Microsoft is not transparent about this (tisk, tisk). But in fact, it uses a type of parametric curve called a Bézier curve, specifically, a third order Bézier curve with 4 control points. It is a type of cubic spline and avoids some of the oscillation problems (http://en.wikipedia.org/wiki/Runge’s_phenomenon) which typically occur when using high degree polynomials for interpolation.

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