library(ggplot2)

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# Example data using mtcars

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a <- aggregate(mpg ~ vs + am , mtcars, function(i) round(mean(i)))

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p <- ggplot(mtcars, aes(factor(vs), y=mpg, fill=factor(am))) + 

            geom_bar(stat="identity",position="dodge") + 

            geom_text(data = a, aes(label = mpg), 

                            position = position_dodge(width=0.9),  size=20)