Where two variables are closely related but move in the reverse direction on a scatter graph, eg when the sales of one product rises, the sales of another might fall. This might occur if the products are substitutes. For example, an estate agent will earn revenue from houses that it rents out to tenants, as well as commission from selling houses. Such is the state of the market that when house sales are rising, the market for renting is less profitable, and vice versa.