  We propose and analyze a multifunctional THz device which can operate as a
tunable switch and a filter. The device consists of a circular graphene
nanodisk coupled to two nanoribbons oriented at $90^\circ$ to each other. The
graphene elements are placed on a dielectric substrate. The nanodisk is
magnetized by a DC magnetic field normal to its plane. The physical principle
of the device is based on the propagation of surface plasmon-polariton waves in
the graphene nanoribbons and excitation of dipole modes in the nanodisk.
Numerical simulations show that 0.61T DC magnetic field provides transmission
(regime ON) at the frequency 5.33 THz with the bandwidth 12.7$\%$ and filtering
with the Q-factor equals to 7.8. At the central frequency, the insertion loss
is around -2 dB and the reflection coefficient is -43 dB. The regime OFF can be
achieved by means of switching DC magnetic field to zero value or by switching
chemical potential of the nanodisk to zero with the ON/OFF ratio better than 20
dB. A small central frequency tuning by chemical potential is possible with a
fixed DC magnetic field.
