Excel has the option under
Tools - Options - Chart tab

Plot empty cells has
- Not plotted (leave gaps)
- Zero
- Interpoloated

i cannot find the option in Excel 2007, after hunting off and on for 3 hours.

Where is it?

i've already dismissed the hack of filling values with the formula
=NA()

References
http://support.microsoft.com/kb/302672
http://j-walk.com/ss/excel/usertips/tip024.htm
http://www.excelforum.com/excel-char...ng-values.html