How are re-transmission and SACK packets displayed in tcptrace graph?

I have two captures (one with SACK and other one without SACK ). From tcptrace graph it is not clear how SACK feature affects the progress of sequence numbers. The graph is very hard to comprehend without any legends. And what are those dark black lines?

SACK enabled:

Without SACK:

P.S: The pictures are zoomed in versions of a larger capture file.