The example in the steps below uses a data window where c0 contains the X values and c1 contains the Y values. A plot has been created and the Y data has been fit with a Smooth curve fit. The steps would be basically the same for any type of fit.
- Once the curve fit is applied, choose the Smooth command from the Curve Fit menu. The Curve Fit Selections dialog will appear.
- Click the View drop-down menu in this dialog and choose Copy Curve Fit to Data Window. This will append two columns to the data window; one for the X values and one for the corresponding Y values. In the example data set described above, these would be located in columns 2 and 3.
- In column 4, enter the known Y values for which you want to find the corresponding X values.
- Run the following formula in Formula Entry: c5=table(c4, c3, c2)
The table function performs a linear interpolation. It looks at the known Y values in c4, looks for the same value in c3, and returns a corresponding X value and stores it in c5.
This process is a bit more difficult if you have a curve (for example a Gaussian curve) that has multiple X values for the same Y value. By masking different sections of the data set each time you run the formula, you would be able to find the different X values.
Posted in: Curve Fitting: Tips