Graphene recently has been demonstrated to support surface-enhanced Raman scattering. Here, we show that the enhancement of the Raman signal of methylene blue on graphene can be tuned by using either the electric field effect or chemical doping. Both doping experiments show that hole-doped graphene yields a larger enhancement than one which is electron-doped; however, chemical doping leads to a significantly larger modulation of the enhancements. The observed enhancement correlates with the changes in the Fermi level of graphene, indicating that the enhancement is chemical in nature, as electromagnetic enhancement is ruled out by hybrid electrodynamical and quantum mechanical simulations.