The curve fit functions in Formula Entry can only find Y values using known X values. The steps listed below can be used to find X values when the Y values are known.
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.