selective imports from grid to avoid duplication of
grid::depth and partykit::depth

updated example/vignette test output

improvements in the plotting functions, especially better x-axis
labeling and streamling of arguments to be mostly consistent with
the corresponding psychotools base functions



git-svn-id: svn://svn.r-forge.r-project.org/svnroot/prefmod@249 edb9625f-4e0d-4859-8d74-9fd3b1da38cb