I know that searching the help files can be a challenge (it seems they don't have a very good search engine), but this one clearly shows, in the format axis dialog (should be the same dialog you used to select the "values in reverse order" option) a

"Horizontal axis crosses at:

automatic
value of
maximum axis value

http://office.microsoft.com/en-us/su...320.aspx?CTT=1

If I understand what is going on, you should just need to select the appropriate option in this dialog.