We consider the scattering of the H-polarized eigenwaves of a planar dielectric waveguide by a coplanar system of graphene strips in the THz range. The strips are placed along the centerline of the waveguide. Our treatment is based on the singular integral equations with the Nystrom-type discretization algorithm. Dependences of the scattering characteristics, near and far fields are studied. Frequency scanning radiation pattern are presented. Maximum of the radiated power is observed near the plasmon resonances. The resonant frequency and main lobe level can be controlled by variation of the chemical potential. Applied optimization procedure allows to obtain the radiation pattern with the side-lobe level less than -20 dB. The presented results can be used in designing of graphene leaky-wave antennas.