The ornamental design for a rockablechair, as shown and described.
FIG. 1 is a front left perspective view of a rockable chair showing my new design;
FIG. 2 is a front right perspective view of the same;
FIG. 3 is a right side view of the same;
FIG. 4 is a left side view of the same;
FIG. 5 is a front elevational view of the same;
FIG. 6 is a back elevational view of the same;
FIG. 7 a top view of the same;
FIG. 8 a bottom view of the same; and,
FIG. 9 is a front left perspective view of the same. The individual, pet animal and pet cushion are shown for environmental purposes only and the broken lines form no part of the claimed design.