layout() {
      const x = d3Hierarchy.treemap();
      x.ratio = _ => {
        const t = x.tile();
        if (t.ratio) x.tile(t.ratio(_));
      };
      x.method = _ => {
        if (has$1(Tiles, _)) x.tile(Tiles[_]);else error('Unrecognized Treemap layout method: ' + _);
      };
      return x;
    }
Treemap layout generator. Adds 'method' and 'ratio' parameters
to configure the underlying tile method.