I have upvoted both of Thomas answers & will be looking into them.
I have done a lot of searching for MSC drawing tools, especially those which take plain text as input. In addition to scripts easy to store in version control, it allows me to code tools (normally as Python scripts), for instance to automatically generate an MSC from a test log to quickly give me a visual overview of a test run's result.
Much as I like the XKCD effect, I have yet to find anything which offers as much control of output possibilities as MSC Generator, which allows diagrams even more complex than this:
Take a look at the examples, to see things like this
There are also powerful nested notations, plus alternatives and option events. Here's a slightly more expressive example:
The program is stable and well supported and has been around for many years. I have found the author to be helpful and open to suggestions. He usually replies to emails within an hour or two.
In addition to the web site and in-built help, I strongly recommend reading the PDF documentation, which seems to be the most complete.
For Windows a full installer is provided, for Linux and MacOS a source tarball is available.
The program is free, and satisfaction is guaranteed, or you get your money refunded in full ;-)