Returns bestfit_function

        args:
            bestfit_x: scalar, array_like
                x value
        return: scalar, array_like
            bestfit y value

def bestfit_func(self, bestfit_x):
        """
        Returns bestfit_function

        args:
            bestfit_x: scalar, array_like
                x value
        return: scalar, array_like
            bestfit y value
        """
        if not self.done_bestfit:
            raise KeyError("Do do_bestfit first")
        bestfit_y = self.fit_args[1] * (bestfit_x ** self.fit_args[0])
        return bestfit_y