excel plot against a date time x series

Why does Excel ignore the time component of datetime values in a chart. For example, the following data :

10/10/2010 9:30   1 
10/10/2010 9:40   2
10/10/2010 9:50   3
10/10/2010 10:00  4 
10/10/2010 10:10  5
10/10/2010 10:20  6

excel plots it all values on a same x-value 10/10/2010. If I format the values as time, it works fine. How do I tell excel to display datetime and interpret it as datetime not date..