consolidating plotting functions into single file

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/xts@863 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
fixing test script and function examples

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/xts@864 edb9625f-4e0d-4859-8d74-9fd3b1da38cb