wi selects data windowing. It is used if you want to fit the data in a certain range of x values. If wi has one argument, it turns windowing on and off. wi 1 turns windowing on. wi 0 turns windowing off. If wi has more than two arguments, windowing is turned on, and the arguments are the minimum and maximum x values used in fitting. In other words, χ2 is computed only for xmin≤x≤xmax.
Examples:
wi 30 100 fit considers only x values between 30 and 100.
wi 30 100 2 10 fit considers only x0 values between 30 and 100 and x1 values between 2 and 10.