The ornamental design for a chairframe, as shown and described.
FIG. 1 is a perspective view of a chair frame showing our new design;
FIG. 2 is an alternative perspective view of FIG. 1; and,
FIG. 3 is another alternative perspective view of FIG. 1.
In the drawings, the dashed lines depict portions of the chair frame, which are environment and form no part of the claimed design; the discontinuous long dashes are shading.