How to quit R script & return to R prompt

You typed a ` without closing it:
barplot(xtab(`profits`,data=Forbes2000)) 

anyway: pushing the escape button should also return you to 
the R-prompt (at least on a Windows platform)

But why is the OP using a backtick at all? It's not necessary in this instance (and in fact it's very rarely necessary at all unless you have used a syntactically invalid name with something silly like spaces in it). See ?Quotes for a little more detail on what ` does.

Also, xtab should possibly be xtabs, and the first argument to that is a one-sided formula, not a vector:

library(HSAUR)
barplot(xtabs( ~ profits,data=Forbes2000)) 

should work fine.

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