I claim the ornamental design for a top plate, as shown and described.
FIG. 1 is a front-left-top perspective view of a top plate showing my new design, the front-right-top, rear-right-bottom and bottom-left-top perspective view being mirror images thereof;
FIG. 2 is a front elevational view thereof, the rear, left side, and right side elevational views being mirror images thereof; and,
FIG. 3 is a top plan view thereof, the bottom plan view being a mirror image thereof.
The broken lines shown in the figures are for the purpose of illustrating boundaries of the design and form no part of the claimed design.