The log trendline equation is y=a*ln(x)+b. Clearly it doesn't allow x
<=0.

OTOH, the exponential trendline equation is y=a*exp(b*x). It can be
transformed into a linear form with ln(y)=ln(a)+b*x. It doesn't allow
y<=0.

You may also want to look at an article I wrote:
Trendline coefficients
http://www.tushar-mehta.com/excel/ti...efficients.htm

--
Regards,

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

In article ,
says...
I have a data set, and when I choose to add a trendline, I can't select the
logarithmic trendline option. Ive tried changing my initial value from 0 to
0.1 in my data set, but this only enables the exponential trendline option.