I have sort of data consist of two variables --dependent and independent-- and want to fit it in linear-plateau form. How to do that in excel or other software and how to identify the inflection point?
I don't use Excel for statistics, but it's difficult for me to imagine fitting a linear plateau model in Excel.
It's relatively easy to do in R or SAS.
For R, you might see the example of linear-plateau here ( https://rcompanion.org/handbook/I_11.html ) (with the caveat that I am the author of this page). You can run R without installing it here ( https://rdrr.io/snippets/ ), although if you are doing analyses at this level, you would likely benefit from getting access and familiarity with statistical software like R or SAS.
You need to install the Solver add-in, then (typically) set up a column of observed values, and a column of calculated values based on the values of Coefficients in separate cells. Use the columns of observed and calculated values to calculate residuals, then a sum-of-squares of these residuals. This is the cell you want to minimise using the Solver, by varying the values of the Coefficient cells.