Sir Cumference and the Viking's Map

Xaxon Yellowbearyd was the fiercest Viking warrior of his time. Now a map to his hidden treasure lies in Radius's and Per's hands. Together the cousins must decode the strange numbered grid on the map—and figure out the secret of the Viking's X and Y axes. As bungling bandits pursue them, Radius and Per use coordinate geometry in their quest for "treasure of the greatest measure."