library(data.table)

 # Generating given dataset
  dat <- data.table(x = sample(c(10, 16, 22, 28, 34, 40), 2000, replace = TRUE),
                   y = sample(seq(0.5, 19.5, by = 1), 2000, replace =  TRUE))

  # Counting how many individuals in each x and add them up
  TempData <- dat[, .(Counting = .N), .(x, y)][order(x, y)]
  TempData <- TempData[, Counting := cumsum(Counting), x]

  # Combine original dataset which in order to acquire the number of
  # counting of individuals
  TempData <- dat[, .(Number = .N), x][TempData, on = "x"]

  # Calculating final required metric
  TempData <- TempData[, result := Counting / Number]
  TempData[, `:=`(Number = NULL, Counting = NULL)]