I have an LG LW65 notebook (widescreen - 1280x800, ATI X700) with the truncated Y axis label problem.
I managed to find a temporary fixing procedure that works for an entire Excel session:

1. close excel
2. change screen resolution to a 4:3 aspect ratio (800x600 or 1024x768)
3. open excel (now the y axis label should display correctly)
4. change screen resolution back to the original value (default)

You shouldn't have any problem while on that excel session. If you close excel, the problem seems to return when you reopen it. You'll have to repeat the procedure.
It's not perfect, but it worked for me.

Good luck!
Francisco