An X11 graphing tool, commonly used in conjunction with tcptrace? ("only" used with it from my experience). It's really simple to use, but one of its most useful features is hidden away and not documented, which is the point of this page.
Xplot can open multiple graphs at once, and use the same axis for all of them, and link all their controls together. What this means is that you can select an area in one graph, and the exact same area is selected in all graphs, making for easy comparison between graphs.
The undocumented command line options to do so are:
No page links to Xplot.
http://www.xplot.org/