Currently, all accessors are a string or number to be used as lookup keys on an array or object (aka the Datum)

const accessor = 'x';
const datum = {
  x: 'a',
};
const x = datum[accessor];

This proposal would introduce the idea of the xAccessor as being a function such as...

const accessor = (d: Datum) => d.x ?? '_all';
const datum = {
  x: 'a',
};
const x = accessor(datum);