changes to crn.plot(). Mostly cosmetic. Well, all cosmetic. But adding chron.plot() as an alias since it makes more sense. Consider making chron an S3Method so that plot(chron(foo)) would work?

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/dplr@745 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
slightly more gracefull axes.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/dplr@746 edb9625f-4e0d-4859-8d74-9fd3b1da38cb