In Excel 2007:

Select the chart you want to adjust,
Then select 'Design' on the main menu bar.
Choose 'Select Data'.
At the bottom of the 'Select Data Source' window displayed is a button labeled 'Hidden and Empty Cells.'
Press that and you should see another window called 'Hidden and Empty Cell Settings.'
This gives you an option to show empty cells as 'Gaps', 'Zero', or 'Connect data points with line.'
An additional tick box in that window allows you to set 'Show data in hidden rows and columns.

Cheers.

William B.

P.S. Anyone discovered how to quickly copy settings from one chart to another in Excel 2007 ???
In Excel 2003 you could just copy a chart with some desired format settings and paste it as
'settings' to another new chart. Which then only required minor corrections to scales or axis
settings. How to do this in 2007 ???