I have researched Power Trend LInes a bit and it looks like most people looking for this feature are utilizing the statistical power of R in conjunction with Tableau. Logistic Regression in Tableau using R « Bora Beran
Bora Beran might have a few additional suggestions.
Currently the easiest solution would be to take log of both fields on x and y axes and fit a linear trend line.
Does that help address your requirement or do you need to render it as a power curve?
With R integration it is possible to fit any curve, I can provide some examples if you need it.