Instantiate barLeft/barRight in Bar Highlighting Section

Was having issues with highlighting and ran across this issue -- https://github.com/flot/flot/issues/1093

Which led me to the commit that fixed this issue so we could patch this on our version.  Worked great, but noticed in the commit (https://github.com/flot/flot/commit/2ce1139cf7ac97c3804c32f30154541a606a19c9) the instantiation for barLeft/barRight was accidentally removed.  This pull request adds back in the instantiation.