Don’t set the this context.

[mbostock]

The nest operator is currently using rollup.call(nest, array) but rollup(array) should be sufficient. I don’t see a good reason to set the nest instance as this. Related:

• nest.rollup uses rollup.call(nest, array) but could use rollup(array).
• extent uses f.call(array, d, i), but could use f(d, i, array).
• max uses f.call(array, d, i), but could use f(d, i, array).
• min uses f.call(array, d, i), but could use f(d, i, array).
• mean uses f.call(array, d, i), but could use f(d, i, array).
• median uses f.call(array, d, i), but could use f(d, i, array).
• sum uses f.call(array, d, i), but could use f(d, i, array).
• variance uses f.call(array, d, i), but could use f(d, i, array).
• map.each uses f.call(map, value, key), but could use f(value, key, map).
• set.each uses f.call(set, value), but could use f(value, value, set). (Note: repeated value!)
• tests!

[mbostock]

Related d3/d3#2246 regarding ES6 arrow functions.