The ornamental design for a frame for a rockingchair, as shown and described.
FIG. 1 is an upper front left perspective view of a frame for a rocking chair showing our new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a right side elevation view thereof;
FIG. 5 is a left side elevation view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is an upper front left perspective view thereof, shown together with a seat cushion, shoulder straps and hip strap in broken lines for the purpose of illustrating an environment in which the frame for a rocking chair may be used but not forming part of the claimed design.