Just now i was tried the following code:
barplot(t(as.matrix(ratop)),beside = T,col=c("red","blue"),
main="Student Score Vs. Top Score",sub="MSCS Semester-1",
xlab="Serial No.",ylab="Percentages",ylim=c(0,100),
legend.text=c("Avg. Score","Top Score"),args.legend=list(x="topright"))
But i got the diagram what i am looking for but i am unable to get the RNO in the X-Axis