graticule.extentMinor = 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 = _;