We claim the ornamental design for an arm-supported flag, as shown and described.
FIG. 1 is a perspective view of an arm-supported flag, showing our new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a left-side elevational view thereof, the fight-side elevational view being a mirror image thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a rear elevational view thereof; and,
FIG. 6 is a bottom plan view thereof.
The broken lines showing a person, logos, another arm-supported flag, and text messages depict environmental matter and form no part of the claimed design.