draw a line at when from y as far as len pixels with attrs.
function (when, len, attrs) {
      var x = time_x(when);
      var end_y = ui.y + len;
      var e = paper.path(
        "M" + x + "," + ui.y + " " +
        "L" + x + "," + end_y
      ).attr(attrs || {});
      return e;    
    }