Draw.io is a a cross-platform open source graphics software for creating diagrams such as flow charts and feedback diagrams. (Link)
Pricing
Anki is free for web, desktop, and Android apps, but the iOS app requires a one-time purchase.
Alternatives

Draw.io has features for linking shapes with arrows (which follow the shape as it is moved), which makes it a great tool for generating images that exhibit some sort of graph-based structure, such as flow charts, feedback diagrams, and circuit diagrams. Diagrams can be edited locally on Windows, Linux, or Mac, or using a web-based app.

When generating diagrams to include into a LaTeX manuscript, I typically export the Draw.io file as a PDF with the diagram embedded in the PDF. That way, I can open and modify the diagram later if all that I have is the PDF graphic file.