Logarithmic Curve Fit

SHAWN18 · Post by **SHAWN18** » Fri May 11, 2007 10:49 am

I have an linear x, y scatter plot. And I am trying to fit a Logarithmic curve to the data. How is this done?

Post by **DPlotAdmin** » Fri May 11, 2007 4:27 pm

The Polynomial Curve Fit command on the Generate menu will (in most cases) use an equation form that matches the scaling on the axes. For example for linear X, linear Y you'll get

y=C0+C1*x+C2*x^2+....

For linear x, logarithmic y you'll get

log10(y)=C0+C1*x+C2*x^2+....

and for logarithmic x, linear y you'll get

y=C0+C1*log10(x)+C2*(log10(x))^2+...

So if that's the form you want, right-click on the graph and select "Logarithmic X, Linear Y".

kapqh4 · Post by **kapqh4** » Wed Jan 14, 2009 5:38 pm

I have searched these forums but I can't find an answer to this. This thread is as close as I've found.

Is there a way to do a natural log curve (base e) instead of a log base 10?

Post by **DPlotAdmin** » Wed Jan 14, 2009 6:45 pm

If your plot's scaling is set to Linear X, Logarithmic Y, then Generate>Polynomial Curve Fit gives you both.

For example:

log(y) = C(0) + C(1)*x + C(2)*x^2 + ...
(log() = common logarithm)
Coefficients
C(00) = 0.346914605862
C(01) = -1.64979805157
C(02) = 3.25720861427

or

ln(y) = D(0) + D(1)*x + D(2)*x^2 + ...
(ln() = natural logarithm)
Coefficients
D(00) = 0.7988004
D(01) = -3.7988004
D(02) = 7.5

On the other hand if you're looking for base e scaling - no, sorry, that isn't in there.