The Time-Line graphs are still EXPERIMENTAL and are under development. The fundamental problem with generating these graphs is that the time values for the segments arriving/leaving are available only from the end where the traffic was captured, while the time values of when the packets arrived or left at the other end have to be estimated with some heuristic. Doing this right tends to be a hard problem taking care of conditions like retransmits, timeouts etc. The current heuristic is a simple one of adding/subtracting 1/3rd of the rtt.
This graph provides a pictorial view of the segments being transmitted in either direction, over the duration of the connection. The Y-axis shows increasing time going from the top to bottom of the graph. The X-axis shows the segments being transmitted between the 2 hosts communicating. As you zoom in with xplot, more and more details will become visible. Here is an example of a time line graph:
Following is a closeup (zoomed in with xplot):
The following features can be seen in the graph
Y-axis : running time of the connection (TOP to BOTTOM, ignoring the negative sign)
TCP Flags (only if set), sequence_number_from:sequence_number_to(difference, bytes transmitted), acknowledgment_sequence_number, advertised window, retransmit indicator (``R''), hardware duplicate indicator (``HD'')
The sequence number for the first segment in either direction is absolute, while the rest are relative to the first segment.