added 2nd plotting scheme in plot2d

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/bayesr@843 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
working on gamma distribution

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/bayesr@844 edb9625f-4e0d-4859-8d74-9fd3b1da38cb