Spotfire makes a distinction between a missing value in a record and a record that is missing altogether. For example, if a record is missing completely (2006 in this example):
Year Category Sales 2005 Bread 58 2007 Bread 24 2008 Bread 43
The line will not break even if "Break lines on empty values" is selected:
But if just a value is missing, but the row for "2006" is present like:
Year Category Sales 2005 Bread 58 2006 Bread 2007 Bread 24 2008 Bread 43
Then the line will break when this option is selected: