  Spectral statistics of weakly-disordered triangular graphene flakes with
zigzag edges are revisited. Earlier, we have found numerically that such
systems may shown spectral fluctuations of GUE, signalling the time-reversal
symmetry breaking at zero magnetic field, accompanied by approximate twofold
valley degeneracy of each energy level [Phys. Rev. B 85, 245424 (2012)].
Atomic-scale disorder induce the scattering of charge carriers between the
valleys and restores the spectral fluctuations of GOE. A simplified description
of such a nonstandard GUE-GOE transition, employing the mixed ensemble of 4x4
real symmetric matrices was also proposed. Here we complement our previous
study by analyzing numerically the spectral fluctuations of large matrices
belonging the same mixed ensemble. Resulting scaling laws relate the ensemble
parameter to physical size and the number of atomic-scale defects in graphene
flake. A phase diagram, indicating the regions in which the signatures of GUE
may by observable in the size-doping parameter plane, is presented.
