http://gnuplot.sourceforge.net/demo/enhancedtext.html
http://gnuplot.sourceforge.net/docs_4.2/node411.html
Gnuplot: bar above letter
It is easy. read the last link above.
Gnuplot: check above letter
set ylabel "{/=28 ~{/CMMI10=34 \022}{1{/CMR10=48 \024}}@^{VS^A}_{12}}"