The ornamental design for a highchair, as shown and described.
FIG. 1 is an upper front left perspective view of a high chair showing our new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a right side elevation view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof; and,
FIG. 7 is an upper front left perspective view thereof, shown without the tray and support frame for the tray as shown in FIG. 1 for convenience of illustration, and shown in an alternate position in which the height of the high chair has been lowered.
The broken lines shown in the drawings are for illustrative purposes only and form no part of the claimed design.