  We analyze numerically ensembles of tight-binding Hamiltonians describing
highly-symmetric graphene nanoflakes with weak diagonal disorder induced by
random electrostatic potential landscapes. When increasing the disorder
strength, statistical distribution of energy levels evolves from Poissonian to
Wigner, indicating the transition to quantum chaos. Power laws with the
universal exponent map the disorder strength in nanoflakes of different sizes,
boundaries, and microscopic disorder types onto a single parameter in additive
random-matrix model.
