You might wish to take a look at Draw.Io:
- Graphical, to use with your mouse: Yes. Easy drag-and-drop :)
- Windows/Mac/Linux, same file format: It's a web-app, so: yes (offline was not a criterium ;)
- Free, ideally open source: Free yes, not sure about the license
Draw.Io is based on mxGraph, which itself is not free – but offers its services for free. You can save created graphs to (and load them from) your local drive, your browsers LocalStorage, Google Drive, or OneDrive. The file format used is based on XML, so you could probably post-process it with your own tools. It supports a load of different graphs: the one you've used in your question is just one of many. The service includes a load of symbols and images to use directly from the page, but also offers to upload and use your own.
To give you a better idea, here are some screenshots:
Draw.Io screenshots (sources: PSWApps Blog, 3Alternatives; click images for larger variants; more screenshots here)
Btw: Draw.Io also allows you to export your work e.g. as PNG (plus in several other formats).