chord.js

function qBPath(sx, sy, tx, ty) {
  var curve = ((tx-sx) * 0.05),
  path = "";
  path += "Q " + (sx) + "," + (sy+curve) + " " + ((sx+tx)/2) +","+ ((sy+ty)/2);
  path += "Q " + (tx) + "," + (ty-curve) + " " + tx + "," + ty;
  return path;
}

path += 'M ' + source_left_offset + ',0'
path += qBPath(source_left_offset, 0, target_x - padding, baseline_y)
path += 'L ' + target_x + padding + ','+baseline_y;
path += qBPath(target_x + padding, baseline_y, source_right_offset, 0)
path += 'Z';