Fixing an assertion error happen when the number of inout equals to and the axs isn't None

src/pymor/models/transfer_function.py

@@ -272,6 +272,8 @@ class TransferFunction(CacheableObject, ParametricObject):
             fig.set_constrained_layout(True)
             ax = fig.subplots(2 * num_output, num_input, sharex=True, squeeze=False)
         else:
+            if num_input == 1:
+                ax = ax.reshape((2*num_output, 1))
             assert isinstance(ax, np.ndarray)
             assert ax.shape == (2 * num_output, num_input), \
                 f'ax.shape={ax.shape} should be ({2 * num_output}, {num_input})'