Adicionar uma Power Trendline parece ter funcionado. tusham gentilmente me salvou o problema de explicar o porquê:
The power trendline is given by y=c*x^b. Believe it or not, you can use LINEST to calculate the parameters. Here's how.
Take the log of the above equation to get ln(y)=ln(c)+b*ln(x). This is a linear equation!
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A log-log display of the data doesn't change the underlying analysis. If you plot a bunch of X,Y pairs and ask for a power trendline, you will get the same result irrespective of whether the plot shows either or both of the axes on a log scale.