For x1 < x < x2, this function interpolates using cubic hermite spline. For x outside this interval, the function linearly extrapolates.
For how to use this function, see BuildingSystems.Utilities.Math.Functions.Examples.CubicHermite.
function cubicHermiteLinearExtrapolation extends Modelica.Icons.Function; input Real x "Abscissa value"; input Real x1 "Lower abscissa value"; input Real x2 "Upper abscissa value"; input Real y1 "Lower ordinate value"; input Real y2 "Upper ordinate value"; input Real y1d "Lower gradient"; input Real y2d "Upper gradient"; output Real y "Interpolated ordinate value"; end cubicHermiteLinearExtrapolation;