range check fixed


git-svn-id: svn://svn.code.sf.net/p/qwt/code/branches/qwt-6.1@1884 29e17f8d-cf37-471e-a288-72cc03a76eda
bounding rect fixed when having only one vertical bar 


git-svn-id: svn://svn.code.sf.net/p/qwt/code/branches/qwt-6.1@1886 29e17f8d-cf37-471e-a288-72cc03a76eda