[ctioga2] fix a ruby1.8 bug

git-svn-id: svn+ssh://rubyforge.org/var/svn/ctioga2/trunk/ctioga2@544 86c91514-0d3b-42e0-aed1-4dfce116d947
[ctioga2] explicit error message upon missing columns for parametric plots (though lines are not taken into account for the time being)

git-svn-id: svn+ssh://rubyforge.org/var/svn/ctioga2/trunk/ctioga2@545 86c91514-0d3b-42e0-aed1-4dfce116d947