I claim the ornamental design for a monitor apparatus, as shown and described.
FIG. 1 is a top, front, right perspective view of the claimed design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a left elevation view thereof;
FIG. 5 is a right elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines in the drawings depict portions of the monitor apparatus that form no part of the claimed design. Portions shown in stipple shading as opposed to line shading show contrasting surfaces unlimited by colors.