A polynomial trend line is a curved line used in graphs to model nonlinear data points. A polynomial trend line will have a different amount of peaks and valleys depending on its order. A second-degree polynomial has a parabolic shape with one main curved change of direction, while a third-degree polynomial has two curves.
Trend lines are used to create an equation that best represents a set of data. This equation is used to interpolate or extrapolate based on the data. Many graphs can only be described with a polynomial equation, which involves one of the variables being raised to a certain order of exponent.