How to format an axis's values — a d3 [format specifier] (https://github.com/d3/d3-format#locale_format) string, or a custom (value) => string function. Omit for the scale's d3 default.
(value) => string
How to format an axis's values — a d3 [format specifier] (https://github.com/d3/d3-format#locale_format) string, or a custom
(value) => stringfunction. Omit for the scale's d3 default.