merge

import std.range : front;

auto xs = ["a", "b"];
auto ys = ["c", "d"];
auto labels = ["e", "f"];
auto aes = Aes!(string[], "x", string[], "y", string[], "label")(xs, ys, labels);

auto nlAes = merge(aes, Aes!(NumericLabel!(string[]), "x",
    NumericLabel!(string[]), "y")(NumericLabel!(string[])(aes.x),
    NumericLabel!(string[])(aes.y)));

assertEqual(nlAes.x.front[0], 0);
assertEqual(nlAes.label.front, "e");