FIG. 1 is a perspective view of the mixing tap showing a new ornamental design thereof; FIG. 2 is a front elevation view of the mixing tap shown in FIG. 1; FIG. 3 is a rear elevation view of the mixing tap shown in FIG. 1; FIG. 4 is a left side elevation view of the mixing tap shown in FIG. 1; FIG. 5 is a right side elevation view of the mixing tap shown in FIG. 1; and, FIG. 6 is a top plan view of the mixing tap shown in FIG. 1.