The ornamental design for a yogamat, as shown and described.
FIG. 1 is a top plan view of a yoga mat showing my new design;
FIG. 2 is a perspective view thereof; and,
FIG. 3 is a perspective view thereof, showing a person stretching on the yoga mat.
The broken line showing text, person, and accessories is included for the purpose of illustrating environmental structure and forms no part of the claimed design.