  We numerically calculate the conductivity $\sigma$ of an undoped graphene
sheet (size $L$) in the limit of vanishingly small lattice constant. We
demonstrate one-parameter scaling for random impurity scattering and determine
the scaling function $\beta(\sigma)=d\ln\sigma/d\ln L$. Contrary to a recent
prediction, the scaling flow has no fixed point ($\beta>0$) for conductivities
up to and beyond the symplectic metal-insulator transition. Instead, the data
supports an alternative scaling flow for which the conductivity at the Dirac
point increases logarithmically with sample size in the absence of intervalley
scattering -- without reaching a scale-invariant limit.
