Pluck multiple variables from a data frame

HTML

<script src="//d3js.org/d3.v3.min.js"></script>

CoffeeScript

data = [
 {a: 1, b: 2, c: 3},
 {a: 2, b: 4, c: 6},
 {a: 3, b: 6, c: 8}
]

keep = ['a']

x = []
data.forEach (d, i) ->
  x_ = d
  for k, v of x_
    if keep.indexOf(k) < 0 then delete x_[k]
  x.push(x_)
console.log(x)
console.log(data)