Fix an annoying special case in log scales

git-svn-id: svn+ssh://rubyforge.org/var/svn/tioga/trunk/tioga@637 f6f669a6-2b79-499e-8f05-be2b991fc3d0
[tioga] fix small a crash and a glitch in showing text label with AT_*_ORIGIN

git-svn-id: svn+ssh://rubyforge.org/var/svn/tioga/trunk/tioga@638 f6f669a6-2b79-499e-8f05-be2b991fc3d0