d3.precisionFixed()

d3.precisionFixed(step)

Returns a suggested decimal precision for fixed point notation given the specified numeric step value. The step represents the minimum absolute difference between values that will be formatted. (This assumes that the values to be formatted are also multiples of step.) For example, given the numbers 1, 1.5, and 2, the step should be 0.5 and the suggested precision is 1:

var p = d3.precisionFixed(0.5),
    f = d3.format("." + p + "f");
f(1);   // "1.0"
f(1.5); // "1.5"
f(2);   // "2.0"

Whereas for the numbers 1, 2 and 3, the step should be 1 and the suggested precision is 0:

var p = d3.precisionFixed(1),
    f = d3.format("." + p + "f");
f(1); // "1"
f(2); // "2"
f(3); // "3"

Note: for the % format type, subtract two:

var p = Math.max(0, d3.precisionFixed(0.05) - 2),
    f = d3.format("." + p + "%");
f(0.45); // "45%"
f(0.50); // "50%"
f(0.55); // "55%"