PlantUML is a nice tool to generate diagram as code but unlike the visual editors (like diagrams.net) you don't have visual feedbacks.

Easy but manual

The following command will generate an image name mydiagram.png

plantuml mydiagram.puml

watch ?

Let's try to update it every 10s with watch. It's a good start but it refresh the diagram even if it's not saved. It you are not lucky it could take ~10s to display the change. (Ok you can lower