A trend line is a graphical representation of a relationship between two variables. It is a straight or curved line that connects data points and shows how those points are related over time, or in relation to another variable. Trend lines can be used to identify patterns, make predictions, and analyze changes in the values being measured. In statistics, trend lines are often used to model relationships between variables, such as the relationship between a company's sales and its advertising expenses, or the relationship between a person's weight and their height. The type of line used depends on the nature of the data: linear regression is used for continuous data while polynomial regression can be used for discrete data.