The ornamental design of a swingarm, as shown and described.
FIG. 1 is a front view of a swing arm for a swing which is shown in broken lines to illustrate the environment of the claimed design and forms no part thereof.
FIG. 2 is a top plan view of the swing arm illustrated in FIG. 1; and,
FIG. 3 is a side view of the swing arm illustrated in FIG. 1.
The bottom view and rear view form no part of the claimed design.