I used Zwibbler in the past in order to do exactly that.
The following is an example of basic diagram I've drawn with this tool:
However, since they shut down their free offer, I mainly used https://www.draw.io
Their tool is free, open-source and quite user-friendly.
You can take a look at their starting templates by clicking on "Create New Diagram" on ...
You simply can't beat MSC generator. It is cross-platform, full featured and well supported (Zoltan generally replies very quickly to emails requesting support or new features).
It takes its input from a plain text file:
Read the excellent user mnaual and search for "loop".