Highly confined surface plasmon polaritons (HC-SPPs) can significantly enhance light-matter interaction and dramatically shrink electronic or photonic systems, including x-ray sources. However, due to the short wavelength and low phase velocity of HC-SPPs, traditional approaches have a hard time exciting them. Here electron-beam excitation of hot carriers plus the quantum Cherenkov effect in graphene are predicted to yield monochromatic HC-SPPs at 1/300 of the vacuum wavelength, with the Fermi energy of graphene dynamically tuning the frequency. Thus one can realize a compact x-ray source without artificial periodic structures, pointing ultimately to tabletop x-ray systems.