  We report observation of graphene plasmon interference fringes across a wide
spectral range using a scattering scanning near-field optical microscope
(s-SNOM) that employs a widely tunable bank of quantum cascade lasers. We use
plasmon interference to measure the dispersion curve of graphene plasmons over
more than an order of magnitude of plasmon wavelength, from $\lambda_{sp}$ ~140
to ~1700 nm, and extract the electron Fermi energy of 298$\pm$4 meV for
hydrogen-intercalated single layer epitaxial graphene on SiC. Furthermore, we
demonstrate the appearance of wavelength tuneable graphene plasmon reflection
"hotspots" at single-layer/bi-layer interfaces. This work demonstrates the
capability of wide-band nano-imaging to precisely measure the electrical
properties of graphene and spatially control plasmon reflection focusing.
