I've got the folloving setup for Mathcad as an OLE object in Excel:
data := csort(data,0)
X := data(0)
Y := data(1)
S := cspline(X,Y)
fit(x) :=interp(S,X,Y,x)
a := fit(x)
Using the folloving VBA-code in Excel I can poke the x-value into the OLE-object and update the calculation, but I cant...