To provide some real world(ish) interpretation, you could consider the following:
Wallace enjoys eating cheese on its own. He doesn't much care for crackers on their own, but he especially loves eating crackers and cheese together, he makes nice little cracker n cheese sandwiches.
In this example, we can think of cheese (x) and crackers (y) as perfect complements, but cheese will be a perfect substitute for a cracker and cheese sandwich.
If a unit of cheese is cheaper than a unit of crackers he'll skip the sandwiches and spend his whole budget on cheese, but if crackers are cheaper he'll buy cheese and crackers in equal proportions.