I claim the ornamental design for a plan storage rack, as shown and described.
FIG. 1 is a perspective view of a plan storage rack, showing my new design;
FIG. 2 is a front elevational view thereof, the rear elevational view being a mirror image thereof;
FIG. 3 is a left-side elevational view thereof, the right-side elevational view being a mirror image thereof;
FIG. 4 is a top plan view thereof; and,
FIG. 5 is a bottom plan view thereof.
The broken lines showing wheels in the figures depict environmental matter and form no part of the claimed design. The remaining broken lines depict tubes and a base of indeterminent depth. The shading on the outside of the cylinders designates a contrasting color to the inside surfaces of the cylinders.