Animation shows a particle (m) moving at v along a straight line. It's angular momentum vector about a fixed point O is shown to be related to the area swept by the radius vector of the particle relative to that fixed point.
The shaded portions are areas swept by the radius vector in equal intervals of time t. Since the velocity is uniform, vt is a constant. The bases of these shaded triangles are equal to Vt each and heights are h (=r sinθ), the perpendicular distance of the line of motion from O. The constancy of angular momentum of the particle about o implies the constancy of areal velocity of radius vector of the paticle about O.
