Hi Jerry,

Here it is. Look forward to your comments.

Option Explicit
Option Base 0
'Function TLcoef(...) returns Trendline coefficients
'Function TLeval(x, ...) evaluates the current trendline at a given x
'
'The arguments of TLcoef, and the last 4 of TLeval: _
vSheet is the name/number of the sheet containing the chart. _
Use of the name (as in the Sheet's tab) is recommended _
vCht is the name/number of the chart. To see this, deselect _
the chart, then shift-click it; its name will appear in the _
drop-down list at the left of formula bar. In the case of a _
chart in its own chartsheet, specify this as zero or the zero _
length string "" _
VSeries is a series name/number, and vTL is the series' trendline _
number. If the series has a name, it is probably better to _
specify the name. To determine the name/number, as well as _
the trendline number needed for vTL, pass the mouse arrow _
over the trendline. Of course, if there is only one series in _
the chart, you can set vSeries = 1, but beware if you add _
more series to the chart.

'First draft written 2003 March 1 by D J Braden _
Revisions by Tushar Mehta (www.tushar-mehta.com) 2005 Jun 19: _
Various documentation changes _
vCht is now 'optional' _
Correctly handles cases where a term is missing -- e.g., _
y = 2x3 + 3x + 10 _
Correctly handles cases where a coefficient is not shown because _
it is the default value -- e.g., y = Ln(x)+10 _
When only the constant term is present, the original function _
returned it in the correct array element only for the _
polynomial and linear fits. Now, the function returns it in _
the correct array element for other types also. For example, _
for an exponential fit, y=10 will be returned as (10,0) _
Arrays are now base zero.
'Limitations: _
The coefficients are returned to precision *displayed* _
To get the most accurate values, format the trendline label _
to scientific notation with 14 decimal places. (Right-click _
the label to do this) _
Given how XL calculation engine works -- recalculates the _
worksheet first, then the chart(s) -- it is eminently _
possible for the chart to show one trendline and the _
function to return coefficients corresponding to the values _
shown by the chart *prior* to the recalculation. To see the _
effect of this '1 recalculation cycle lag' plot a series of _
random numbers. _
An alternative to the functions in this module is the LINEST _
worksheet function. Except for those few cases where LINEST _
returns incorrect results, it is the more robust function _
since it doesn't suffer from the '1 recalculation cycle' _
lag. With XL2003 LINEST may even return more accurate _
results than the trendline.

Function TLcoef(vSheet, vCht, vSeries, vTL)
'To get the coefficients of a chart on a chartsheet, specify vCht _
as zero or the zero length string ""

'Return coefficients of an Excel chart trendline. _
Limitations: See the documentation at the top of the module _
'Note: For a polynomial fit, it is possible the trendline doesn't _
report all the terms. So this function returns an array of _
length (1 + the order of the requested fit), *not* the number of _
values displayed. The last value in the returned array is the _
constant term; preceeding values correspond to higher-order x.
Dim o As Trendline
Application.Volatile
If ParamErr(TLcoef, vSheet, vCht, vSeries, vTL) Then Exit Function
On Error Resume Next
If vCht = "" Or vCht = 0 Then
If TypeOf Sheets(vSheet) Is Chart Then
Set o = Sheets(vSheet).SeriesCollection(vSeries) _
.Trendlines(vTL)
Else
TLcoef = "#Err: vCht can be omitted only if vSheet is a " _
& "chartsheet"
Exit Function '*****
End If
Else
Set o = Sheets(vSheet).ChartObjects(vCht).Chart. _
SeriesCollection(vSeries).Trendlines(vTL)
End If
On Error GoTo 0
If o Is Nothing Then
TLcoef = "#Err: No trendline matches the specified parameters"
Else
TLcoef = ExtractCoef(o)
End If
End Function


Function TLeval(vX, vSheet, vCht, vSeries, vTL)
'DJ Braden
'Exp/logs are done for cases xlPower and xlExponential to _
allow for greater range of arguments.
Dim o As Trendline, vRet

Application.Volatile
If ParamErr(TLeval, vSheet, vCht, vSeries, vTL) Then Exit Function
On Error Resume Next
If vCht = "" Or vCht = 0 Then
If TypeOf Sheets(vSheet) Is Chart Then
Set o = Sheets(vSheet).SeriesCollection(vSeries) _
.Trendlines(vTL)
Else
TLeval = "#Err: vCht can be omitted only if vSheet is a " _
& "chartsheet"
Exit Function '*****
End If
Else
Set o = Sheets(vSheet).ChartObjects(vCht).Chart. _
SeriesCollection(vSeries).Trendlines(vTL)
End If
On Error GoTo 0
If o Is Nothing Then
TLeval = "#Err: No trendline matches the specified parameters"
Exit Function
End If
vRet = ExtractCoef(o)
If TypeName(vRet) = "String" Then TLeval = vRet: Exit Function
Select Case o.Type
Case xlLinear
TLeval = vX * vRet(LBound(vRet)) + vRet(UBound(vRet))
Case xlExponential 'see comment above
TLeval = Exp(Log(vRet(LBound(vRet))) + vX * vRet(UBound(vRet)))
Case xlLogarithmic
TLeval = vRet(LBound(vRet)) * Log(vX) + vRet(UBound(vRet))
Case xlPower 'see comment above
TLeval = Exp(Log(vRet(LBound(vRet))) _
+ Log(vX) * vRet(UBound(vRet)))
Case xlPolynomial
Dim Idx As Long
TLeval = vRet(LBound(vRet)) * vX + vRet(LBound(vRet) + 1)
For Idx = LBound(vRet) + 2 To UBound(vRet)
TLeval = vX * TLeval + vRet(Idx)
Next Idx
End Select
End Function

Private Function DecodeOneTerm(ByVal TLText As String, _
ByVal SearchToken As String, _
ByVal UnspecifiedConstant As Byte)
'splits {optional number}{SearchToken} _
{optional numeric constant}
Dim v(1) As Double, TokenLoc As Long
TokenLoc = InStr(1, TLText, SearchToken, vbTextCompare)
If TokenLoc = 0 Then
v(1) = CDbl(TLText)
Else
If TokenLoc = 1 Then v(0) = 1 _
Else v(0) = Left(TLText, TokenLoc - 1)
If TokenLoc + Len(SearchToken) Len(TLText) Then _
v(1) = UnspecifiedConstant _
Else v(1) = Mid(TLText, TokenLoc + Len(SearchToken))
End If
DecodeOneTerm = v
End Function
Private Function getXPower(ByVal TLText As String, _
ByVal XPos As Long)
If XPos = Len(TLText) Then
getXPower = 1
ElseIf IsNumeric(Mid(TLText, XPos + 1, 1)) Then
getXPower = Mid(TLText, XPos + 1, 1)
Else
getXPower = 1
End If
End Function

Private Function ExtractCoef(o As Trendline)
Dim XPos As Long, s As String
On Error Resume Next
s = o.DataLabel.Text
On Error GoTo 0
If s = "" Then
ExtractCoef = "#Err: No trendline equation found"
Exit Function '*****
End If
If o.DisplayRSquared Then s = Left$(s, InStr(s, "R") - 2)
s = Trim(Mid(s, InStr(1, s, "=", vbTextCompare) + 1))
Select Case o.Type
Case xlMovingAvg
Case xlLogarithmic
ExtractCoef = DecodeOneTerm(s, "Ln(x)", 0)
Case xlLinear
ExtractCoef = DecodeOneTerm(s, "x", 0)
Case xlExponential
s = Application.WorksheetFunction.Substitute(s, "x", "")
ExtractCoef = DecodeOneTerm(s, "e", 1)
Case xlPower
ExtractCoef = DecodeOneTerm(s, "x", 1)
Case xlPolynomial
Dim lOrd As Long
ReDim v(o.Order) As Double
s = Application.WorksheetFunction.Substitute(s, " ", "")
s = Application.WorksheetFunction.Substitute(s, "+x", "+1x")
s = Application.WorksheetFunction.Substitute(s, "-x", "-1x")
Do While s < ""
XPos = InStr(1, s, "x")
If XPos = 0 Then
v(o.Order) = s 'constant term
s = ""
Else
lOrd = getXPower(s, XPos)
If XPos = 1 Then v(UBound(v) - lOrd) = 1 _
Else _
v(UBound(v) - lOrd) = Left(s, XPos - 1)
If XPos = Len(s) Then
s = ""
ElseIf IsNumeric(Mid(s, XPos + 1, 1)) Then
s = Trim(Mid(s, XPos + 2))
Else
s = Trim(Mid(s, XPos + 1))
End If
End If
Loop
ExtractCoef = v
End Select
End Function
Private Function ParamErr(v, ParamArray parms())
Dim l As Long
For l = LBound(parms) To UBound(parms)
If VarType(parms(l)) = vbError Then
v = parms(l)
ParamErr = True
Exit Function
End If
Next l
End Function

--
Regards,

Tushar Mehta
www.tushar-mehta.com
Excel, PowerPoint, and VBA add-ins, tutorials
Custom MS Office productivity solutions

In article ,
says...
As I recall, Braden's code also required that the graph be on a
worksheet, and would only extract the displayed precision.
Unfortunately, the chart trendline remains Excel's most stable and
accurate least-squares platform (hopefully that will change with the
next version of Excel). I would be glad to put your code through its
paces if you so desire.

Jerry

Tushar Mehta wrote:

Hi Jerry,

Do be aware that while Dave Braden's code might be the best option for
the job at hand, it has limitations. If I remember correctly, one of
them is that if a coefficient is 1, XL doesn't show it and the code
fails to handle that correctly. I have an improved version on some
computer but haven't shared it with the world because it has been tested
very lightly and, of course, XL2003 came along.