code: function (bbox, buffer) {
          var v = buffer / this.ky;
          var h = buffer / this.kx;
          return [
              bbox[0] - h,
              bbox[1] - v,
              bbox[2] + h,
              bbox[3] + v
          ];
      }
docs: Given a bounding box, returns the box buffered by a given distance.
  
  @param {Array<number>} box object ([w, s, e, n])
  @param {number} buffer
  @returns {Array<number>} box object ([w, s, e, n])
  @example
  var bbox = ruler.bufferBBox([30.5, 50.5, 31, 51], 0.2);
  //=bbox