polygon methods for lineplots

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/directlabels@676 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
use function instead of dl.trans to avoid NOTE

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/directlabels@677 edb9625f-4e0d-4859-8d74-9fd3b1da38cb