docs: def x(self):
          '''
          np.array: The grid points in x.
          '''
          if None not in (self.x_min, self.x_max, self.x_step) and \
                  self.x_min != self.x_max:
              x = np.arange(self.x_min, self.x_max+self.x_step-self.y_step*0.1, self.x_step)
          else:
              x = np.array([])
          return x