df <- data.frame(a=runif(15000000), b = rnorm(15000000))
 ggplot(data=df, aes(x=a, y=b)) + geom_density2d()

Error : cannot allocate vector of size 11.2 Gb
In addition: Warning messages:
1: In outer(gx, x, "-") :
  Reached total allocation of 16365Mb: see help(memory.size)
2: In outer(gx, x, "-") :
  Reached total allocation of 16365Mb: see help(memory.size)
3: In outer(gx, x, "-") :
  Reached total allocation of 16365Mb: see help(memory.size)
4: In outer(gx, x, "-") :
  Reached total allocation of 16365Mb: see help(memory.size)