DPlot

This would be for interactive use only, as I could see allowing for programming it might be complex.

In large data sets I sometimes want to do a curve fit of a piece of a curve. Now, I would have to export the curve to another file and truncate the ends as desired, then work with the piece.

What I would like is the possibility of limiting the endpoints in the initial calculation of the curve fit, just as you allow the limitation of the calculated fit. Then I can keep everything together in one file.

The polynomial fit is of the most interest to me right now. I see the "More Curve Fits" plugin may have some of this capability perhaps, but you have to put in the initial values, it seems. It also doesn't seem to have the accuracy to handle the date and time values that I find myself using all of the time.

...I see the "More Curve Fits" plugin may have some of this capability perhaps, but you have to put in the initial values, it seems.

For several of the equation forms (particularly those with a variable exponent), yes you do. For polynomials no - you should always converge to a good solution no matter what the initial values are.

It also doesn't seem to have the accuracy to handle the date and time values that I find myself using all of the time.

Very possible. I'll look into this.