This took me an inordinately long time to figure out, so I'll document it here. When writing a Vega description of a chart and including a tooltip, the accessor methods for datum cannot be used inside of format(). For example, this doesn't work (it always displays 'NaN'):
"tooltip": {"signal": "{title: datum.name, 'CO2eq': format(datum.size, '.2f') + ' Gigatons'}"}
This does:
"tooltip": {"signal": "{title: datum.name, 'CO2eq': format(datum['size'], '.2f') + ' Gigatons'}"}
Note the difference inside the format(), using datum['size'] instead of datum.size.