gtable

The R package gtable is designed to help construct and manipulate layouts containing graphical elements. The standard grid package in R provides low-level functions to define viewports, and place graphical elements (grobs) at specific locations within the device window. gtable builds upon these functions and provides a higher-level interface, where one can e.g. merge two layouts, add columns, rows, insert graphical elements in a given cell, and change the display order, among other things.

The gtable package is used internally by ggplot2, and can therefore be used to modify the layout of such plots.