Allowable trendlines 1

[corus]

I have about 8 pairs of x,y data I want to fit a trendline to. The options available though are only for linear, polynomial, exponential, and moving average. The power and logarithmic options are greyed out. Why should that be?

corus

 

[MintJulep]

Don't know.

Try using the LINEST() or LOGEST() functions directly on the underlying data rather than the trendline feature of charts. Excel's help sort of explains how to use LINEST to achieve higher order polynomial fits.

Both functions are array functions.

 

[Drew08]

Power function is linear Log vs Log(x) plot, Log function is a linear y vs Log(x) plot. The Log(0) = infinity, and the Log(-x) does not exist. Therefore, if your data contains either 0 or negative numbers, you cannot use the power or log functions.

Try shifting the data by a constant(C) so the lowest data point is +1. Then, the plotted log function is y = m Log(x+C)+b and plotted power function is Log(y+C1)= m Log(x+C2)+b

Or, if an entire series is negative, multiply by -1 so the plotted log function is y = m Log(-x)+b and plotted power function is Log(-y) = m Log(-x)+b

 

[corus]

Many thanks. As you said, the data contains x=0. Changing that to 1e-6 allows multi-types of curve fit.

corus