The fp command turns the use of the fitplot.exe program on and off. The fitplot.exe program is a fast plotting program which is used to plot the data and attempted fit every iteration. This is useful if you want to see what the Levenberg-Marquardt algorithm is doing. The use of fitplot.exe is off by default, although if there are any command line arguments to fit2.exe, the use of fitplot.exe is turned on.