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I think Dia, which is a general diagram editor, checks most or all of your boxes. http://dia-installer.de/
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/34627 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/34627 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/