These will only return linear interpolations,

no, that isn't correct.

If you want to graph your data, you can access the equation produced.

Tushar Mehta:
Trendline coefficients
http://www.tushar-mehta.com/excel/ti...efficients.htm



You can see how Bernard Liengme solves for the coefficients of a polynomial
curve using LINEST.
http://www.stfx.ca/people/bliengme/E...Polynomial.htm

==========================================

Chip Pearson posted some code to get the values from the chart itself:

http://groups.google.com/groups?thre...GP09.p hx.gbl

From: "Chip Pearson"
References:
Subject: Trendline Data
Date: Thu, 19 Aug 2004 13:28:02 -0500
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If you mean to get the polynomial coefficients, try something
like the following:

Dim TL As Trendline
Dim A As Double, B As Double, C As Double, D As Double, _
E As Double, F As Double, G As Double
Dim S As String
Dim Arr As Variant
Dim Pos As Integer
Set Ser = Chart1.SeriesCollection(1)
Set TL = Ser.Trendlines.Add(xlPolynomial, 6) ' change 6 to
appropriate order
TL.DisplayEquation = True



TL.DataLabel.NumberFormat = "0.000000000000000"
S = TL.DataLabel.Text
Pos = InStr(1, S, "=")
S = Mid(S, Pos + 1)
S = Replace(S, " + ", "|+")
S = Replace(S, " - ", "|-")
Arr = Split(S, "|")

A = 0: B = 0: C = 0: D = 0: E = 0: F = 0: G = 0
' g
Pos = InStr(1, Arr(LBound(Arr) + 0), "x")
G = CDbl(Left(Arr(LBound(Arr) + 0), Pos - 1))

' f
Pos = InStr(1, Arr(LBound(Arr) + 1), "x")
F = CDbl(Left(Arr(LBound(Arr) + 1), Pos - 1))

' e
Pos = InStr(1, Arr(LBound(Arr) + 2), "x")
E = CDbl(Left(Arr(LBound(Arr) + 2), Pos - 1))

' D
Pos = InStr(1, Arr(LBound(Arr) + 3), "x")
D = CDbl(Left(Arr(LBound(Arr) + 3), Pos - 1))

' c
Pos = InStr(1, Arr(LBound(Arr) + 4), "x")
C = CDbl(Left(Arr(LBound(Arr) + 4), Pos - 1))

' b
Pos = InStr(1, Arr(LBound(Arr) + 5), "x")
B = CDbl(Left(Arr(LBound(Arr) + 5), Pos - 1))

' a
A = CDbl(Arr(UBound(Arr)))


Cordially,
Chip Pearson


--
Regards,
Tom Ogilvy






"JAG-W" wrote in message
ups.com...

Tom Ogilvy wrote:
If you want to do a linear interpolation, then use the Forecast
function.
FORECAST(x,known_y's,known_x's)

=FORECAST(2,{10,20},{1,3})
returns 15 for example.

Also look at LINEST

These will only return linear interpolations, I was hoping to be able
to some how to access the trendline formulae that the charting function
has and use that, but this seems not to be possible :(

Best

JAG-W