Applet shows a particle (m) moving at v along a straight line. It's angular momentum vector about a fixed point is shown to be related to the area swept by the radius vector of the particle relative to that fixed point.
The red shaded areas shown are areas swept by the radius vector in time t. Since the velocity is uniform, vt is a constant. The bases of these triangles are equal to Vt each and heights are h (r sin ?), 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.