Found it.  in ajgraph.c starting at line 2760:
    xa[0] = x;
    ya[0] = y + incr;
    xa[1] = x + incr;
    ya[1] = y + size;
    xa[2] = x + size;
    ya[2] = y + incr;
    xa[3] = x + incr;
    ya[3] = y;
    xa[4] = x;
    ya[4] = y;
the last line should be
    ya[4] = y + incr;
Regards,
David Mathog
mathog at caltech.edu
Manager, Sequence Analysis Facility, Biology Division, Caltech