In response to a recent comment, I have modified the CSpline2 spreadsheet so that the CSplineA and CHSplineA functions will accept x values list in descending order, as well as ascending.  For more details of the background to the functions, and how to use them see: Daily Download 22: Splines and Curves.  The new version can be downloaded from:

CSpline2.zip

The code for the checking and reversing the x data is quite short:

Function CheckAscX(Xa As Variant, Ya As Variant, n as Long) As Boolean
Dim Temp As Variant, i As Long
' If last x < first x, reverse Xa and Ya and return True, else return False
    If Xa(n, 1) < Xa(1, 1) Then
        Temp = Xa
        For i = 1 To n
            Xa(i, 1) = Temp(n - i + 1, 1)
        Next i
        Temp = Ya
        For i = 1 To n
            Ya(i, 1) = Temp(n - i + 1, 1)
        Next i
        CheckAscX = True
        Exit Function
    End If
    CheckAscX = False
End Function

Then the CheckAscX function just needs to be called from the spline function:

...
    Xa = GetArray(Xa)
    Ya = GetArray(Ya)

    n = UBound(Xa)
    RevX = CheckAscX(Xa, Ya, n)
...

The spline functions will now work with the X values sorted in either direction: