The ornamental design for a violin, as shown and described.
FIG. 1 is a front perspective view of a violin according to our new design;
FIG. 2 is a back perspective view thereof;
FIG. 3 is an enlarged front view thereof;
FIG. 4 is an enlarged side view thereof;
FIG. 5 is an enlarged opposing side view thereof;
FIG. 6 is an enlarged back view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a top view thereof.
The broken lines showing portions of a violin are included for the purpose of illustrating environmental structure and form no part of the claimed design. The broken lines shown cutting away the top of the violin in FIGS. 3, 4, 5, and 6 show where the enlarged views are cut and forms no part of the claimed design.