Cubic Spline Series Function
The Cubic Spline series function is used to interpolate the data in the
source series. Piecewise third-order polynomial is used to do the interpolation.
The second derivative of each polynomial is set to zero at the two endpoints. This series function takes a single source series.
The user has the option to specify the number of interpolation points for
each piece. This is done by specifying the Frequency Factor on the Cubic
Spline tab-page of the Series
Function dialog box.