Mscgen is a nifty tool for generating sequence diagrams. One feature I miss in Mscgen is the ability to add notes within the sequence diagram. I use some of the existing features of mscgen to add notes. Sample output is below.
The tricks/hacks used are
Use an invisible entity for notes. The linecolor is set to white and label set to empty to prevent the entity from showing up in the output.
A15 [label="A15"], m4 [label="M4"], notes [linecolor="white",label=""];
Create a box within the notes entity with the desired annotation. Set the line color to white to hide the box boundary.
notes box notes [label="M4 executes in parallel with A15 loading Linux", linecolor="white"];
Place the annotation box at the same level as the desired operation.
A15 -> A15 [label="Start loading\nLinux"], m4 box m4 [label="Start Video Capture\nand\nDisplay"], notes box notes [label="M4 executes in parallel with A15 loading Linux", linecolor="white"];
The link to the full source is here
It is easy to do live updates with mscgen in discussions due to
iwatch to monitor the filesystem for any changes and running mscgen on any modified files. I use
eog displaying the output as it reflects any image file changes immediately, offering a live view.