Use formatter to add plus sign in table visualization

[William]

Is it possible to use a formatter to get plus signs to appear before positive values? It's a very simple matter to add a plus sign using "prefix", but how do I make that conditional on the number being positive?

[William]

I've worked this out now -- I needed to put this in options, which follows the same format (the ICU pattern set) I would use in Excel:

pattern: '+#0; -#0; 0'