area.topline, area.baseline?

[mbostock]

Something like this:

d3.area.prototype.topline = function() {
  return d3.line().x(this.x()).y(this.y1());
};

d3.area.prototype.baseline = function() {
  return d3.line().x(this.x()).y(this.y0());
};

Although, I feel like we’d want something symmetric for x and vertically-oriented areas.

[mbostock]

Maybe:

d3.area.prototype.lineY1 = function() {
  return d3.line().x(this.x()).y(this.y1());
};

d3.area.prototype.lineY0 = function() {
  return d3.line().x(this.x()).y(this.y0());
};

d3.area.prototype.lineX0 = function() {
  return d3.line().x(this.x0()).y(this.y());
};

d3.area.prototype.lineX1 = function() {
  return d3.line().x(this.x1()).y(this.y());
};

Should also copy area.curve, area.context and area.defined.