draw a horizontal line from the time offsets start to end with attrs. If start or end isn't set, use the left and right margins respectively. y_adj can be used to adjust y.
function (start, end, y_adj, attrs) {
      var start_x = time_x(start) || margin[3];
      var end_x = time_x(end) || ui.w - margin[1];
      var y = ui.y + y_adj;
      var e = paper.path(
        "M" + start_x + "," + y + " " +
        "L" + end_x + "," + y
      ).attr(attrs || {});
      return e;
    }