graticule.majorExtent = function(_) {
      if (!arguments.length) return [ [ X0, Y0 ], [ X1, Y1 ] ];
      X0 = +_[0][0], X1 = +_[1][0];
      Y0 = +_[0][1], Y1 = +_[1][1];
      if (X0 > X1) _ = X0, X0 = X1, X1 = _;