The environment variables must be defined before gnuplot is executed; how to do so depends on your operating system.
FIT_LOG
changes the name (and/or path) of the file to which the fit log will be written from the default of "fit.log" in the working directory.
FIT_SCRIPT
specifies a command that may be executed after an user interrupt. The default is replot, but a plot or load command may be useful to display a plot customized to highlight the progress of the fit.