Function curve

explainshell/web/static/js/d3.v3.js:6993–6995

(r0, p0, r1, p1)

      return "A" + r + "," + r + " 0 " + +(a > π) + ",1 " + p;
    }
    function curve(r0, p0, r1, p1) {
      return "Q 0,0 " + p1;
    }
    chord.radius = function(v) {
      if (!arguments.length) return radius;
      radius = d3_functor(v);