docs: 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