fix tutorials to avoid a crash when plotting chain and FC scanned points  (see https://savannah.cern.ch/bugs/index.php?94761)
fix tutorials to avoid a crash when plotting chain and FC scanned points 
(see https://savannah.cern.ch/bugs/index.php?94761)


git-svn-id: ecbadac9c76e8cf640a0bca86f6bd796c98521e3@45720 27541ba8-7e3a-0410-8455-c3a389f83636

bbannier/ROOT,bbannier/ROOT,bbannier/ROOT,bbannier/ROOT,bbannier/ROOT,bbannier/ROOT,bbannier/ROOT