The ornamental design for a monitor, as shown and described.
FIG. 1 is a perspective view of a monitor, showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left-side view thereof;
FIG. 5 is a right-side view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof;
FIG. 8 is a rear perspective view thereof; and,
FIG. 9 is another right-side view thereof, showing another viewing position.
The broken lines are for environmental purposes only and form no part of the claimed design. The shade lines in the figures show contour and not surface ornamentation. The diagonal lines in FIGS. 1, 2, and 6 represent a reflective surface.