I don't understand this graph. From the graph, does it mean that at high pressures the oxide layer is stable and this stability reduces at low pressures and temperatures?
The graph does show that at high pressures the $\Delta G$ per surface area values are lower, which would usually indicate stability. I agree with you that as the pressure goes lower, the stability reduces.
However I disagree regarding temperature: As the temperatures decrease (from the green curve denoting 1500K to the blue curve denoting 300K), the $\Delta G$ values are going down, meaning an increase in stability.