The tooltips for black areas contain "undefined%". You might want to display "no data" in these cases instead: format = d => d != null ? `${d}%` : 'no data'

Thanks. I implemented it in the code to check if `data.get(d.id)` is undefined and changed the message if it is. I couldn't use your accessor code since the d3 bound data contains the FIPS code only. The actual data is in a Map object where FIPS maps to the number. So I have to use (data.get(d.id)===undefined) ? `${d}%` : 'no data'

The code was meant for the "format" cell. Sorry that I didn't make that clear.