We claim the ornamental design for a machine monitor, as shown and described.
FIG. 1 is a perspective view of a machine monitor showing our new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is front view thereof;
FIG. 4 is a; left side view thereof;
FIG. 5 is a rear view thereof;
FIG. 6 is bottom plan view thereof; and,
FIG. 7 is the right side view of the monitoring device.
The structural features depicted by means of broken lines in the drawing figures have been shown for the purpose of illustrating environmental structure only and form no part of the claimed design.