The following page in the documentation
http://www.sagemath.org/doc/reference/sage/graphs/graph.html
refers to the need to install GLPK, and that chromatic_number uses the less efficient DLX algorithm by default, as opposed to the MILP algorithm.

It's my understanding that sage now includes GLPK by default.

I'd recommend that the current version (4.8) documentation be changed to remove the sentence:
"This method requires you to install an optional Sage package like GLPK or COIN-OR’s CBC."

Additionally, we should check for other references to installing "optional" GLPK in the documentation

Component: documentation

Keywords: chromatic_number

Author: Joal Heagney, Nathann Cohen

Reviewer: Karl-Dieter Crisman

Merged: sage-5.0.beta3

Issue created by migration from https://trac.sagemath.org/ticket/12378