You can use the keyword :NA: where a variable name or number would normally appear. :NA: takes on a special value (-2109 ) that can be used to test for the existence of data. "Data Equations" below gives an example of how to test for the existence of data at a given time using :NA:.
If you assign a variable the value :NA: at any time the value will not appear in graphs (it will be ignored or interpolated over) and will show up as only "—" in tables.
