Created attachment 85812 [details]
Example file

Problem description: 
When I create a chart with a date axis, the axis is reset to a string data type when I save and re-open the file.

Steps to reproduce:
1. create a new file with the following data:

Date	Number
01-01-13	1
23-01-13	2
30-01-13	3

2. Create a line chart (it automatically creates a date axis)

3. Notice the line is not straight (as expected)

3. Save the file

4. Restart LibreOffice

5. Re-open the file

Current behavior:

The date axis is now a "text" type axis. Notice the line is straight.

Expected behavior:

File reproduced exactly as it was saved

Other notes:

- I remember this worked correctly in a pre-4.0 release. I am not sure which.
- I use the NL version of LibreOffice
Operating System: Windows 7
Version: 4.1.1.2 release
Last worked in: 3.6.7.2 release