In a 2-dimensional

Cartesian coordinate system In geometry, a Cartesian coordinate system (, ) in a plane (geometry), plane is a coordinate system that specifies each point (geometry), point uniquely by a pair of real numbers called ''coordinates'', which are the positive and negative number ...

, with ''x'' representing the

abscissa In mathematics, the abscissa (; plural ''abscissae'' or ''abscissas'') and the ordinate are respectively the first and second coordinate of a point in a Cartesian coordinate system: : abscissa \equiv x-axis (horizontal) coordinate : ordinate \eq ...

and ''y'' the

ordinate In mathematics, the abscissa (; plural ''abscissae'' or ''abscissas'') and the ordinate are respectively the first and second coordinate of a point in a Cartesian coordinate system: : abscissa \equiv x-axis (horizontal) coordinate : ordinate \e ...

, the identity line or line of equality is the ''y'' = ''x'' line. The line, sometimes called the 1:1 line, has a

slope In mathematics, the slope or gradient of a Line (mathematics), line is a number that describes the direction (geometry), direction of the line on a plane (geometry), plane. Often denoted by the letter ''m'', slope is calculated as the ratio of t ...

of 1. When the abscissa and ordinate are on the same scale, the identity line forms a 45° angle with the abscissa, and is thus also, informally, called the 45° line. The line is often used as a reference in a 2-dimensional

scatter plot A scatter plot, also called a scatterplot, scatter graph, scatter chart, scattergram, or scatter diagram, is a type of plot or mathematical diagram using Cartesian coordinates to display values for typically two variables for a set of dat ...

comparing two sets of data expected to be identical under ideal conditions. When the corresponding data points from the two data sets are equal to each other, the corresponding scatters fall exactly on the identity line. In

economics Economics () is a behavioral science that studies the Production (economics), production, distribution (economics), distribution, and Consumption (economics), consumption of goods and services. Economics focuses on the behaviour and interac ...

, an identity line is used in the

Keynesian cross The Keynesian cross diagram is a formulation of the central ideas in Keynes' '' General Theory of Employment, Interest and Money''. It first appeared as a central component of macroeconomic theory as it was taught by Paul Samuelson in his textboo ...

diagram to identify equilibrium, as only on the identity line does aggregate demand equal aggregate supply..

References

Coordinate systems Statistical charts and diagrams Economics curves {{Statistics-stub