The ornamental design for a coverplate, as shown and described.
FIG. 1 is a top perspective view of a cover plate;
FIG. 2 is a front view of a cover plate;
FIG. 3 is a back view of a cover plate;
FIG. 4 is a right side view of a cover plate;
FIG. 5 is a left side view of a cover plate;
FIG. 6 is a top view of a cover plate; and,
FIG. 7 is a bottom view of a cover plate.
The broken lines showing portions of the design in all figures is included for the purpose of illustrating environment and unclaimed features and forms not part of the claimed design.