This Instructional Activity is a part of the Lesson 5, Unit 5, Grade 8. The graph of a function is the set of all points with an input for the function and its matching output. How did you use this definition to determine which graphs, if any, were functions? By the definition of a function, we need each input to have only one output. On a graph, this means there cannot be two different ordered pairs with the same value. Assume the following set of ordered pairs is from some graph.