I was reading this article on Landau theory https://phys.libretexts.org/Bookshelves/Thermodynamics_and_Statistical_Mechanics/Book%3A_Thermodynamics_and_Statistical_Mechanics_(Arovas)/07%3A_Mean_Field_Theory_of_Phase_Transitions/7.05%3A_Landau_Theory_of_Phase_Transitions
I fail to understand how did the author plot the phase diagram on the $(a, y)$ plane. Could someone help?
