changes for non-equally spaced time based x-axis

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/xts@856 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
minor adjustment to y-label offset

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/xts@857 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
adding args for addLegend

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/xts@858 edb9625f-4e0d-4859-8d74-9fd3b1da38cb