* ext/tk/lib/tkextlib/tcllib/plotchart.rb: fix to invoke correct function
  of tcllib. Patch by @zalt50 [fix GH-787]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49034 b2dd03c8-39d4-4d8f-98ff-823fe69b080e