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' -- You received this message because you are subscribed to the Google Groups "Google Visualization API" group. To unsubscribe from this group and stop receiving emails from it, send an email to google-visualization-api+unsubscr...@googlegroups.com. To post to this group, send email to google-visualization-api@googlegroups.com. Visit this group at https://groups.google.com/group/google-visualization-api. To view this discussion on the web visit https://groups.google.com/d/msgid/google-visualization-api/f384f517-2af8-453c-9a5b-5e5c6beea7d9%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.