I'm trying to use Excel to create a trendline for data that approximates a shifted-power law equation, of the form y=a*(x-b)^c.

The built-in formulae in Excel 2007 aren't able to give adequate results, so I would really like to find a method within Excel of obtaining the coefficients a, b and c.

At the moment I'm stuck with two scenarios - one unacceptable, and the other horribly inelegant, and prone to fail:

1) export data to a third-party application, and try and obtain coefficients from there - not acceptable, since I'm trying to create a spreadsheet that will produce the result of manipulating the curve simply from a single user-input (i.e. the graph information itself)

2) brute-force estimating generic ranges for a, b and c and calculating formula results from inputting these into the formula, and performing a least-squares fit on the resulting data compared with the original - inelegant, since I have coarse arrays of 20 elements in each of a, b and c, yielding thousands of datapoints; and prone to fail, since the estimated arrays could well not be appropriate for the input data

Is there a formula-based solution to this problem? Failing which, is there a VBA solution that will be able to produce coefficients for a shifted-power law equation for any data input (obviously assuming reasonable input data).

I'd appreciate your help - and please ask if I've been unclear, or more information is required.

Thanks

PatrickW