I've used GraphViz a number of times and highly recommend it as a standard tool on your belt. Having a stand-alone executable that can export to SVG is great.

The most complex thing I've done with it [1]: a tool (MIT-license) that builds diagrams of the data and addressing pipeline for a DSP processor, and lets one 'scrub through' the assembler code frame by frame and see the values propagate through the blocks.

Also PlantUML [2] uses it for most diagrams.

Getting layout and positioning the way you want can be tricky but is usually achievable with patience and hidden objects.

[1] https://github.com/paphillips/DFB [2] https://plantuml.com/graphviz-dot