The aim of this test is:
A chamber filled with ideal gas is sealed tightly with a movable, elastic piston. The position of the piston is varied between different load steps. Friction between the piston and the chamber wall is neglected. For simplicitly, also initially the elastic piston is in an unstressed state.
The figure above shows that the piston is being compressed ($y$ displacement has larger negative values at the top) by the forces acting on it. The initial position of the top part of the piston is indicated as a wireframe.
The plot shows that the relation between the stress in the piston and its displacement coincides with the pressure-volume relation of the chamber. That indicates that the Jacobian computation inside the Python BC classes was correctly implemented.