The ornamental design for an adjustablechair, as shown and described.
FIG. 1 is a top perspective view of an adjustable chair showing our new design.
FIG. 2 is a right side elevational view thereof.
FIG. 3 is a left side elevational view thereof.
FIG. 4 is a top plan elevational view thereof.
FIG. 5 is a front elevational view thereof; and,
FIG. 6 is a back elevational view thereof.
The broken lines showing the chair base are unclaimed subject matter and form no part of the claimed design. The dashed lines on the padding of the chair show a stitching pattern and are part of the claimed design.