We report that the behavior of a low-energy π plasmon excitation in a single layer graphene (SLG) can be modified by doping external potassium (K) atoms, a feature demanded to realize the graphene plasmonics. Using high-resolution electron-energy-loss spectroscopy, we find that upon K-doping the π plasmon energy increases by 1.1 eV due to the K-induced electron density up to n = 7 × 1013 cm−2 in SLG. The four modified dispersions for different K-dopings, however, are found to merge into a single universal curve when plotted in the dimensionless coordinates indicating that the unique plasmonic character of SLG is preserved despite the K-dopings.