The ornamental design for a chairframe, as shown and described.
FIG. 1 is a front, left side perspective view of a chair frame comprising my new design;
FIG. 2 is a left-side perspective view thereof;
FIG. 3 is a front-elevation view thereof;
FIG. 4 is a rear-elevation view thereof;
FIG. 5 is a side-elevation view, the opposite side elevation view being a mirror image thereof; and,
FIG. 6 is a top-plan view thereof.
The dashed lines depicting the chair seat and backrest in FIG. 1 have been shown for illustrative purposes only and form no part of the claimed design.