In diagram edit mode, the save button is functionnal but allows you to save a "chart document" (ODC). The user can think that the spreadsheet is saved. Moreover, if you close the document, no question will be asked (save before close). So there is a big risk to loose a work.  

Steps to reproduce 
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1. Create 2 columns of numbers (for example) and don't save your file.
2. Select the cells and create a XY chart (for example).
3. Verify that you are still in diagram edit mode (no data menu). If not, double click on the chart to enter this mode.
4. Click on the save button, give a name and click save. This creates an ODC file.
5. Exit Calc and see that no question is asked to save the document.
6. Now you have a chart file, but the spreadsheet is lost!

Expected behaviour
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1. A saved chart should not be interpreted by Calc as a saved spreadsheet. If the spreadsheet has been modified since the last save (or is new), Calc MUST ask to save it before close (even if the chart inside has been saved).
2. The save button action in diagram edit mode is confusing. Ideally, this button should always save the whole spreadsheet and the chart save feature should be somewhere else.