How do I fit a function to data?

Access the Plot: Fitting tab of the Graph Options dialog box, select the appropriate plot, and click the Add new fit button; then, select the desired type of function or smoothing procedure in the Fit type box. You can adjust the fitting options (e.g., stiffness or optimization settings) and the pattern for the graphical representation of the fit on that tab as well. The pattern can also be adjusted by double-clicking on the fit line or surface in the graph. (See also, How do I fit a custom-defined function to the data?).