from ase.phasediagram import PhaseDiagram

ref = [('K', 0), ('Ta', 0), ('O2', 0),
       ('K3TaO8', -16.167), ('KO2', -2.288),
       ('KO3', -2.239), ('Ta2O5', -19.801),
       ('TaO3', -8.556), ('TaO', -1.967),
       ('K2O', -3.076), ('K2O2', -4.257),
       ('KTaO3', -13.439)]

pd = PhaseDiagram(ref)
pd.plot()