A nonlinear graph is a graph that depicts any function that is not a straight line; this type of function is known as a nonlinear function. Linear functions, or those that are a straight line, display relationships that are directly proportional between an input and an output while nonlinear functions display a relationship that is not proportional.
A nonlinear graph shows a function as a series of equations that describe the relationship between the variables. The nonlinear system of equations provides the constraints for this relationship. Nonlinear graphs can show curves, asymptotes and exponential functions.
An example of something that a nonlinear graph would depict is population growth. An area's population starts off small, but it then begins to grow exponentially as more members are added.