How to put labels over geom_bar for each bar in R with ggplot2

To add to rcs' answer, if you want to use position_dodge() with geom_bar() when x is a POSIX.ct date, you must multiply the width by 86400, e.g.,

ggplot(data=dat, aes(x=Types, y=Number, fill=sample)) + 
 geom_bar(position = "dodge", stat = 'identity') +
 geom_text(aes(label=Number), position=position_dodge(width=0.9*86400), vjust=-0.25)

Try this:

ggplot(data=dat, aes(x=Types, y=Number, fill=sample)) + 
     geom_bar(position = 'dodge', stat='identity') +
     geom_text(aes(label=Number), position=position_dodge(width=0.9), vjust=-0.25)