The ornamental design for an anti-fatigue mat, as shown and described.
FIG. 1 is a perspective view of an anti-fatigue mat showing my design;
FIG. 2 is a top view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a front elevation view thereof;
FIG. 5 is a rear elevation view thereof; and,
FIG. 6 is a side elevation view thereof, the opposite side elevation being identical.
The broken lines showing of a pattern formed by a series of horizontal bars are for purpose of illustrating environmental structure and forms no part of the claimed design.