TITLE: Software to find a function for data approximation
QUESTION [10 upvotes]: I've got some y(x) 2D data set. I would like to find a function fitting this data:

Is there any open source or free software to find a function to approximate a data sequence like the above?
Here is what I found so far:

Fityk is a program for data processing and nonlinear curve fitting. The program can be used for any task that requires fitting a curve to 2D (x,y) data.
Simfit is a free software OpenSource Windows/Linux package for simulation, curve fitting, statistics, and plotting, using a library of models or user-defined equations.
Zunzun.com website. Here you can curve fit and surface fit your 2D and 3D data online with a rich set of error histograms, error plots, curve plots, surface plots, contour plots, VRML, auto-generated source code, and PDF file output.

REPLY [3 votes]: zunzun does not seem to be available any longer.  However, I found http://www.mycurvefit.com to be a useful alternative for trying different models.  This includes linear, non-linear regression, splines and custom models.