The ornamental design for a swivelchair, as shown and described.
FIG. 1 is a perspective view of a swivel chair showing my new design.
FIG. 2 is a front elevational view thereof.
FIG. 3 is a rear elevational view thereof.
FIG. 4 is a left side elevational view thereof.
FIG. 5 is a right side elevational view thereof.
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken line showing of the “HAG” logo in FIG. 3 is for the purpose of illustrating environmental structure and forms no part of the claimed design.