  Using low-temperature high-magnetic-field scanning tunneling microscopy and
spectroscopy (STM/STS), we systematically study a graphene quantum dot (GQD)
defined by a circular graphene p-p junction. Inside the GQD, we observe a
series of quasi-bound states arising from whispering-gallery-mode (WGM)
confinement of the circular junction and directly visualize these quasi-bound
states down to atomic dimensions. By applying a strong magnetic field, a large
jump in energy of the quasi-bound states, which is about one-half the energy
spacing between the quasi-bound states, is observed. Such a behavior results
from turning on a {\pi} Berry phase of massless Dirac fermions in graphene by a
magnetic field. Moreover, our experiment demonstrates that a quasi-bound state
splits into two peaks with an energy separation of about 26 meV when the Fermi
level crosses the quasi-bound state, indicating that there are strong
electron-electron interactions in the GQD.
