d3.nest()
  .key(function(d) { return d.year; })
  .map(data).each(function(values, key) { return 0;});

map.each(function(v, k) { array.push({key: k, values: entries(v, depth)});

each: function(f) {
    for (var property in this) if (property[0] === prefix) f(this[property], property.slice(1), this);
  }