merge fix 48950 from Sven in roostats development branch
"Fix plotting in certain situations for importance sampling"


git-svn-id: https://root.cern.ch/svn/root/branches/v5-34-00-patches@49058 27541ba8-7e3a-0410-8455-c3a389f83636
fix for bug https://savannah.cern.ch/bugs/index.php?100561


git-svn-id: https://root.cern.ch/svn/root/branches/v5-34-00-patches@49060 27541ba8-7e3a-0410-8455-c3a389f83636
tiny doc update


git-svn-id: https://root.cern.ch/svn/root/branches/v5-34-00-patches@49061 27541ba8-7e3a-0410-8455-c3a389f83636