For example, you could look at the daily sales of ice cream and the daily high temperature as the two variables at play in a graph and find a crude linear relationship between the two.

Graphs of two or more straight lines can be used to solve simultaneous linear equations. The graph of a straight line can be described using an equation close equationA mathematical statement ...

Nancy R. Mann, Optimum Estimators for Linear Functions of Location and Scale Parameters, The Annals of Mathematical Statistics, Vol. 40, No. 6 (Dec., 1969), pp. 2149-2155 ...