I claim the ornamental design for a portion of a bag holder, as shown and described.
FIG. 1 is a perspective view of a portion of a bag holder, showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a right-side elevational view thereof;
FIG. 5 is a left-side elevational view thereof;
FIG. 6 is a rear elevational view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines showing a portion of the bag holder, a bag and a support structure are for the purposes of illustrating environment and form no part of the claimed design.