function stringify (data) {
  if (typeof data !== 'object') { return data; }
  return [data.x, data.y, data.z, data.w].join(' ').trim();
}
Stringify coordinates from an object with keys [x y z].
Example: {x: 3, y: 10, z: -5} to "3 10 -5".

@param {object|string} data - An object with keys [x y z].
@returns {string} An "x y z" string.