The Nonlinear Curve Fit option in the Curve Fitter button menu in the Data Table toolbar will determine the best fit from your choice of 15 built-in parametric functions plus one user-defined equation. These parametric nonlinear equations are divided into four families: Standard, Waveform, Peak, and Transition. For more detail on the 15 resident parametric functions, please refer to Appendix A. You may also include the “User-Defined” selection as one of your functions. See the section on User-Defined Curve Fit.

## Generate and Plot a Nonlinear Curve Fit

- Create a new graph by clicking
**Graphs →****Add New Graph…**in TechGraph Online’s navigation bar. - In the
**TechGraph Editor**, click on the**X Data (Label)**header and choose*X Data Type:*“Number” and*Graph Type:*“Scatter”. - Enter the following data into the Data Table:

- Click the
**Curve Fitter**button and select**Nonlinear Curve Fit**from the dropdown list. The Nonlinear Curve Fit menu will appear. Let’s assume that we know our data has a trend like a sigmoidal curve. Select the Sigmoidal (#9) box and then enter “X” in the*X Data Col*field, “A” in the*Y Data Col*field, “1” in*Start Row*, and “11” in*End Row*.

- Click
**Calculate**. After a successful fit calculation, the Coefficient of Determination for the fitted sigmoidal equation is shown at the bottom of the menu (0.998846 in this example). - Select the
**Save**tab in the Nonlinear Curve Fit menu. Select “Y Predict Equation” from the*Selection*list. Enter “A” in the*Destination Col*field and click**Save**. A curve fit equation is generated and saved in the Graph A*Equation*field of the Curves and Error Bars menu. - Switch to the
**Edit Chart**tab and the resulting chart will be displayed with the data and the Sigmoidal equation drawn.

**Note**: The sigmoidal equation is initially shown in the middle of the chart. Select the equation and grab the middle bubble to move the equation to a new location on the chart.

When you edit the data values associated with the Nonlinear Curve Fit equation, you will need to redo steps 4 to 6 to get the updated coefficient values of the equation.

Refer to Tutorial 3: Linear Curve Fit Graphs for instructions on how to change the equation style (color, line thickness, fill color).