[Bug 123824] New: Exponential curve fit y=a*exp(b*x) ignores data points with zero y-values

[bu...@apache.org]

https://issues.apache.org/ooo/show_bug.cgi?id=123824

            Bug ID: 123824
        Issue Type: DEFECT
           Summary: Exponential curve fit y=a*exp(b*x) ignores data points
                    with zero y-values
           Product: General
           Version: 4.0.1
          Hardware: PC
                OS: Windows 7
            Status: UNCONFIRMED
          Severity: critical
          Priority: P3
         Component: chart
          Assignee: issues@openoffice.apache.org
          Reporter: forster@uw.edu

Created attachment 82063
  --> https://issues.apache.org/ooo/attachment.cgi?id=82063&action=edit
spreadsheet with charts to demonstrate y=exp(x) problem

When choosing an exponential trend line y=a*exp(b*x) through some actual data I
realized that a number of the data point that had zero y-values were being
ignored.  This is different from a reported bug that described a curve fit not
going through a data point at the origin.  Although no exponential regression
of the form y=a*exp(b*x) can intersect the x-axis, there should be no reason
why a minimization of least square values between data and candidate
regressions should have a problem.  The attached spread sheet demonstrates the
problem.  I did not try to demonstrate what a data point within the x-range of
data with a nonzero y-value would do, but the example shown is enough to
demonstrate a problem.  Note the example was created and showed the problem
before it was saved in .xls format for the first time for whatever that is
worth.

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[Bug 123824] Exponential curve fit y=a*exp(b*x) ignores data points with zero y-values

[bu...@apache.org]

https://issues.apache.org/ooo/show_bug.cgi?id=123824

--- Comment #2 from Fred Forster <fo...@uw.edu> ---
Quite right, and if Excel is disabled with any y=0 data points, that is the
safe things to do.  It appears Chart just ignores any such data points, does
not alert the user and merrily goes on to calculate an incorrect curve.  This
could be addressed with an alert or at least stated somewhere that the user
needs to translate his/her own data so all x and y values are positive for
valid results. Not knowing the philosophy behind OpenOffice, it is not clear if
it is supposed to produce the same answers as Excel, or do things better when
appropriate.  Anyway, I fired up Systat's SigmaPlot and got done what I needed.
 I believe it just picks initial value for coefficients and uses something like
a method of steepest ascent to minimize least square differences, but that is
just a guess.

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[Bug 123824] Exponential curve fit y=a*exp(b*x) ignores data points with zero y-values

[bu...@apache.org]

https://issues.apache.org/ooo/show_bug.cgi?id=123824

Regina Henschel <rb...@t-online.de> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |rb.henschel@t-online.de
         Resolution|---                         |WONTFIX

--- Comment #1 from Regina Henschel <rb...@t-online.de> ---
With the current algorithm it is not possible to use such data pairs, because
the fitting is not done on the equation y=a*exp(b*x) but on the equation
ln(y)=ln(a)+ b*x.
Excel has disabled exponential trend lines in those cases.

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