  We explore the longitudinal conductivity of graphene at the Dirac point in a
strong magnetic field with two types of short-range scatterers: adatoms that
mix the valleys and "scalar" impurities that do not mix them. A scattering
theory for the Dirac equation is employed to express the conductance of a
graphene sample as a function of impurity coordinates; an averaging over
impurity positions is then performed numerically. The conductivity $\sigma$ is
equal to the ballistic value $4e^2/\pi h$ for each disorder realization
provided the number of flux quanta considerably exceeds the number of
impurities. For weaker fields, the conductivity in the presence of scalar
impurities scales to the quantum-Hall critical point with $\sigma \simeq 4
\times 0.4 e^2/h$ at half filling or to zero away from half filling due to the
onset of Anderson localization. For adatoms, the localization behavior is
obtained also at half filling due to splitting of the critical energy by
intervalley scattering. Our results reveal a complex scaling flow governed by
fixed points of different symmetry classes: remarkably, all key manifestations
of Anderson localization and criticality in two dimensions are observed
numerically in a single setup.
