I am very fond of using graphviz (https://graphviz.org/) to draw the finite state machines that my programs interact with. For example, this is a description of the state machine inside the Linux kernel audio subsystem, as it was several years ago: https://source.chromium.org/chromium/chromium/src/+/master:c... You can draw it using ‘xdot’ for something interactive or with ‘dot’ for a static png or svg file.