library(ggplot2)

  # Sample Data
  dat <- data.table(gender=as.factor(sample(c("M", "F"), 500, TRUE)),value=rnorm(500))

  # key is gender
  setkey(dat, gender)

  A.DT[gender=="M",  gp + geom_density(data=.SD)]
  A.DT[.("M"),  gp + geom_density(data=.SD)]