Just to be sure, did you look at the existing trend line options? I'm thinking the polynomial option might be what you are looking for.
If those don't work, you can build your own via table calculations or the R integration that is new in 8.1 and add them on a secondary axis.
Thank you for all the links!!!!
The power trend is different from the polynomial one. I will try installing R, which I haven't used beforen but seems interesting.
Do you know the R-script to use for power fit? Coding seems not so straightforward.
Power trendlines are now supported by Tableau 10.5