I claim the ornamental design for octagon weight plate, as shown and described.
FIG. 1 is a front plan view of an octagon weight plate, the rear being a mirror image thereof;
FIG. 2 is a side elevational view of the octagon weight plate, each side being a mirror image thereof;
FIG. 3 is a side view of the octagon weight plate mounted on a bar, each side being a mirror image thereof;
FIG. 4 is an isometric view of an octagon weight plate, each side being a mirror image thereof; and,
FIG. 5 is an isometric view of an octagon weight plate mounted on a bar, each side being a mirror image thereof.
The broken lines on the octagon weight plate are for the purpose of illustrating portions of the article that form no part of the claim. The broken line showing of the bar is for the purpose of illustrating environment and forms no part of the claim.