Add any string suffix to number.

[inesmoskal]

I know there is the possibility to add the currency suffix, but it would be useful to be able to add a more flexible suffix. For example "°C" for temperature values or "kg".

[mbostock]

For a fixed suffix, the recommended way is to wrap the format function. For example:

const formatNumber = d3.format("d"); 
const formatCelsius = d => `${formatNumber(d)}°C`;

You can also combine the two like so:

const formatCelsius = (f => d => `${f(d)}°C`)(d3.format("d"));

We could perhaps formalize this pattern as:

const formatCelsius = d3.format("d", (f, d) => `${f(d)}°C`);

Or maybe make the arguments order-dependent to allow pre- (e.g., multiplying by 100) and post-formatting:

const formatCelsius = d3.format("d", s => `${s}°C`);

[mbostock]

This was secretly added in #135. You can now pass an options second argument to d3.format that accepts a suffix (and prefix). For example:

d3.format("d", {suffix: "°C"})(25) // "25°C"

The advantage of this approach over the above wrappers is that it works correctly with respect to some of the more advanced formatting directives, such as parentheses for negative numbers.

d3.format("(d", {suffix: "°C"})(-25) // "(25°C)"