Graphic Presentation: Line Graphs and Economic Curves

Line Graphs

A line graph plots successive values of a variable — most often against time — as points on a coordinate system, and joins the points with straight-line segments. It is the natural choice for showing a trend: how national income, price level, or population has moved year after year.

Economic Curves

Many relationships that Economics studies are not "one value per year" but "one value of Y for every possible value of X" — the demand curve (quantity demanded at every possible price), the supply curve (quantity supplied at every possible price), average-cost and marginal-cost curves, the Lorenz curve, the Phillips curve, and so on. When such a relationship is plotted, the resulting line — straight or curved — is usually called a curve in economics, whether or not it is actually curved in the geometric sense. A "demand curve" can, in fact, be a perfectly straight line if the underlying demand function is linear (Section 7).

An Important Convention: Which Axis Gets Which Variable …