# Chapter 3. Coordinate Systems and Axes

To make any sort of data visualization, we need to define position
scales, which determine where in a graphic different data values are
located. We cannot visualize data without placing different data points
at different locations, even if we just arrange them next to each other
along a line. For regular 2D visualizations, two numbers are required to
uniquely specify a point, and therefore we need two position scales.
These two scales are usually but not necessarily the *x* and *y* axes of
the plot. We also have to specify the relative geometric arrangement of
these scales. Conventionally, the *x* axis runs horizontally and the *y*
axis vertically, but we could choose other arrangements. For example, we
could have the *y* axis run at an acute angle relative to the *x* axis,
or we could have one axis run in a circle and the other run radially.
The combination of a set of position scales and their relative geometric
arrangement is called a *coordinate system.*

# Cartesian Coordinates

The most widely used coordinate system for data visualization is the 2D
*Cartesian coordinate system*, where each location is uniquely specified
by an *x* and a *y* value. The *x* and *y* axes run orthogonally to each
other, and data values are placed in an even spacing along both axes
(Figure 3-1). The two axes are continuous position scales, and they can represent both positive and negative real numbers. To fully specify the coordinate system, we need to specify the range of numbers ...

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