You can use an IF formula with NA(). For example:

=IF(OR(B2="",B2=0),NA(),B2-A2)

The chart will interpolate over the #N/A errors. To hide these errors,
you can use conditional formatting.
http://www.contextures.com/xlCondFormat03.html#Errors


Excel Charts and Formulas wrote:
If a formula is in the cell but is blank or equals "0", how do you get a
chart not to plot until a number appears?


--
Debra Dalgleish
Excel FAQ, Tips & Book List
http://www.contextures.com/tiptech.html