def y(self):
        '''
        np.array: The grid points in y.
        '''
        if None not in (self.y_min, self.y_max, self.y_step) and \
                self.y_min != self.y_max:
            y = np.arange(self.y_min, self.y_max-self.y_step*0.1, self.y_step)
        else:
            y = np.array([])
        return y
np.array: The grid points in y.