non linear regressions and histograms.

[Jean Bréfort <jean brefort normalesup org>]

Hi,

I have made some work on non linear regression curves in graphs, and
especially in histograms. As a sample, I implemented a gaussian fit. The
patch is available, for interested people, at:
http://jean.brefort.free.fr/histograms-regs.patch (~30kB).

There are probably other distributions that it might be interesting to
implement in the same way (any suggestion?).

I do not know if R2 can be evaluated for such a regression, and even
what it would mean, so it is not enabled in that case.
I also fixed go_non_linear_regression to make it work. BTW, should we
keep the same code in gnumeric (current situation) or should gnumeric
use the functions in go-regression.c?

Waiting for your comments and suggestions.

Cheers,
Jean