Use enum

git-svn-id: http://primefaces.googlecode.com/svn/primefaces/trunk@10995 5a70e874-a5cb-11dd-8af2-493fb0bb9f7c
Fixed category rendering for linechart

git-svn-id: http://primefaces.googlecode.com/svn/primefaces/trunk@10997 5a70e874-a5cb-11dd-8af2-493fb0bb9f7c