We claim the ornamental design for a dial of a compass, as shown and described.
FIG. 1 is top, side perspective view of a dial of a compass; and,
FIG. 2 is a top view of the dial of FIG. 1.
The ornamental design that is claimed is shown in solid lines in the drawings. The broken (dashed) lines shown in FIGS. 1 and 2 are included for the purpose of illustrating environmental elements only and form no part of the claimed design.