Calc plot axis format is not saved

LibreOffice version (but same behavior in earlier releases)
Windows 7

I have several XY plots in LibreOffice Calc spreadsheets. I set a particular format for the numbers on the using edit > format axis. But after saving the file and closing it, the numeric format has switched back to Source Format. How can I make it stay with the format I set?
Also, lately the y-axis label sometimes switches to 0 degree rotation rather than the 90 degree rotation (text parallel to the axis), which is how it was created and saved.
I have the data on one tab (sheet), the plot on another tab (sheet).
The spreadsheet is saved in Excel format, either .xls or .xlsx (both Excel formats have the same behavior.)

What happens if you save in ods-format?