Haven't Excel available currently, but it looks like your graph gets 0 for your date value. Probably the date you have as source is really a text string - e.g. a result of FORMAT function. The numeric value of any text is 0.

About date and time. A date is really an integer, which presents the number of days from start of previous century. And time is a number, where Integer part of time value represents the number of full days (24 Hrs=1), and fraction part represents hours, minutes, and seconds. E.g. when you format a number 0.25 as time, you get 6:00:00.