I’m trying to work out a way to auto-generate message flow diagrams from method calls or traces in log files. Here I found a paper that shows how to do that for TCP/IP network traffic logs.

Message Sequence Chart (MSC) diagrams were designed for modeling of
message-based communication between entities in the network. They are
used primarily by developing of a new network protocol as a
description of design requirements. However, MSC can be as well used
for visualization of real traffic captured from a network. One of the
applications aimed at work with MSC formalism is Sequence Chart Studio
(SCStudio), which is being developed in research centre Institute for
Theoretical Computer Science at FI MUNI.

The thesis is focused on the problem of generating Message Sequence
Chart (MSC) diagrams from PCAP (packet capture) file format. This
functionality was integrated into SCStu- dio as a new feature for
importing PCAP files. This paper describes the developing process and
behaviour of implemented functionality.

Other sequence diagram generators:

• TraceUML to generate sequence diagrams from log statements.
• seqdiag is a Python tool.
• mscpackage is a LaTeX package for drawing message sequence diagrams that supports the full MSC2000 language.
• Quick Sequence Diagram Editor is a tool for creating UML sequence diagrams from textual descriptions of objects and messages.
• JS Sequence Diagrams is a simple javascript library to turn text into vector UML sequence diagrams.
• Jumly is a JavaScript library for UML diagram rendering engine. It works completely on client side.

I’m currently using Mscgen because I can edit and render diagram source in Emacs with org-mode.