Plotting of chemical-potential diagrams (CPDs)

[FPrith]

I have two questions on the plotting of chemical-potential diagrams (CPDs).

(Question 1) For unary systems (one-component systems), what kind of the format of target_vertices.yaml files are required?
I have done DFT calculation and defect analysis on diamond-type Si.
As a test, I modified my target_vertices.yaml file by hand.
My concern is especially on the settings of "chem_pot:" and "competing_phases:"; the empty setting of "competing_phases:" is problematic or not.

target: Si
A:
chem_pot: Si: 0
competing_phases: -
impurity_phases: []

(Question 2) For the Sc-N-Mg ternary system, the CPD was plotted as the Sc-N binary system. When no ternary compounds are included, will pydefect create a binary CPD?
I have done DFT calculation and defect analysis on rock-salt-type Mg-doped ScN, which was reported by your paper.
https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.9.034019
Fot plotting the CPD of the Sc-N-Mg ternary system, the total energies of tightly relaxed cells of almost all of the compounds in this system that were reported experimentally (ScN, Sc, Mg, N2, MgSc, Mg3N2) (MgSc2 was excluded in my test).

As described in the pydefect manual, the CPD of the Mg-Al-O ternary system created via pydefect is ternary; this was confirmed in my test!
On the other hand, in the case of the Sc-N-Mg ternary system, its CPD was described as the Sc-N binary system.