How does one denote that a cell value is not available / doesn't make sense in a table? I have a table that compares the performance of different systems according to different error measures:

System   | Error 1 | change | Error 2 | ...
--------------------------------------| ...
Baseline | 0.6     | -      | ...
A        | 0.1     | -0.5   | ...
B        | 0.3     | -0.3   | ...
C        | 0.7     |  0.1   | ...
D        | 0.6     |  0.0   | ...


I currently chose - to denote that the value does not make sense. Of course, I can subtract the error of the baseline system from itself and get 0. But I don't think that makes sense.

Should I put - in that cell? Or -- or --- (I write the document with LaTeX) or eventually something different like N/A? Or make the cell black?
If it is self-evident (remember to look at your table from the perspective of a reader) I would just leave the cell empty. Less clutter is better. When not, I would tend towards --. More importantly I would add a footnote to the table explaining what -- means. I think the footnote is in that case more important than the difference between -, --, and ---.