code: def show_distance_matrix(self):
          
          distance_matrix = self.get_distance_matrix()
          
          plt.imshow(distance_matrix, cmap = plt.get_cmap('hot'), interpolation='kaiser')
          plt.title("U-Matrix")
          plt.colorbar()
          plt.show()
docs: !
          @brief Shows gray visualization of U-matrix (distance matrix).
          
          @see get_distance_matrix()