set.seed(42)
    quantity <- c(runif(100, 0, 100))
    dat <- data.frame(
      qty = quantity,
      qtile = cut(quantity, quantile(quantity, seq(0, 1, 0.1)),
      include.lowest = TRUE))
    dat <- dat %>% group_by(qtile) %>% summarise(qty = sum(qty))
    ggplot(dat, aes(qtile, qty)) + geom_bar(stat = 'identity')

set.seed(42)
quantity <- c(runif(100, 0, 100))

dat <- data.table(
  qty = quantity,
  wt = quantity
)
dat[!is.na(qty), avg := sum(wt) / 10]
setorder(dat, qty, wt)
dat[!is.na(qty), cum_wt := cumsum(wt)]
dat[!is.na(qty), level := cum_wt / avg]
dat[!is.na(qty), qtile := ceiling(level)]

dat <- dat[, .(qty = sum(qty)), by = 'qtile']

ggplot(dat, aes(qtile, qty)) + geom_bar(stat = 'identity')