code: function arcPath(g, o) {
      var x = o.x || 0,
          y = o.y || 0,
          ir = o.innerRadius || 0,
          or = o.outerRadius || 0,
          sa = (o.startAngle || 0) - Math.PI/2,
          ea = (o.endAngle || 0) - Math.PI/2;
      g.beginPath();
      if (ir === 0) g.moveTo(x, y);
      else g.arc(x, y, ir, sa, ea, 0);
      g.arc(x, y, or, ea, sa, 1);
      g.closePath();
    }
docs: path generators