  The {\pi}-plasmon dispersion in graphene was scrutinized by
momentum(q)-resolved electron energy-loss spectroscopy with an improved q
resolution and found to display the square root of q dispersion characteristic
of the collective excitation of two-dimensional electron systems, in contrast
with previous experimental and theoretical studies which reported a linear q
dispersion. Our theoretical elaborations on the q-dependent spectra affirm this
square root of q relation and further unveil an in-plane electronic anisotropy.
The physical property of the {\pi} plasmon is thoroughly compared to that of
the two-dimensional plasmon due to carriers of the Dirac fermions. A clear
distinction between the {\pi} plasmon and the two-dimensional Dirac plasmon was
demonstrated, clarifying the common notion on correlating the
linearly-dispersed Dirac cones to the linear dispersion of the {\pi} plasmon
previously reported.
