The following code is an example of how to draw a graphic on every page of a document. The graphic (a grey rectangle) appears in a different place for each chapter, and the chapter number appears in the rectangle as well. This is often called a thumb index - your thumb can flip the pages of a book while you watch the contents flash past...
An example (MkIV) for using Metapost to create thumb indices for chapters.
<texcodecontext source="yes">% these first lines are just for wiki layout and visualisation\setuppapersize[A8][A4]\showframe\setuppaper[A4nx=4,oversizedny=4]\setuparranging[XY]\usemodule[visual]