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  5. Tutorial 4: Nonlinear Curve Fit Graphs

Tutorial 4: Nonlinear Curve Fit Graphs

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

  1. Create a new graph by clicking Graphs → Add New Graph… in TechGraph Online’s navigation bar. 
  2. In the TechGraph Editor, click on the X Data (Label) header and choose X Data Type: “Number” and Graph Type: “Scatter”.
  3. Enter the following data into the Data Table:
  1. 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.
  1. 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).  
  2. 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.
  3. 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).

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