The coordinate system adjusts the mapping from coordinates to the 2D plane of the computer screen. Among the different coordinate systems available in ggplot2, the Cartesian system is the most common coordinate system for two dimensions, while the polar coordinate system is often used for special plots, such as pie charts. When you create a plot, the coordinate system for the graph will be set with default values, which, in most cases, would be Cartesian coordinates. If you want a different coordinate system, you can overwrite the default value using the appropriate function. Such functions have the general form coord_x, where x is replaced by the specific coordinate desired.

The following is a table summarizing the main functions of coordinate systems; a more exhaustive list can be found on the package website: