d3.precisionRound()

d3.precisionRound(step, max)

Returns a suggested decimal precision for format types that round to significant digits given the specified numeric step and max values. The step represents the minimum absolute difference between values that will be formatted, and the max represents the largest absolute value that will be formatted. (This assumes that the values to be formatted are also multiples of step.) For example, given the numbers 0.99, 1.0, and 1.01, the step should be 0.01, the max should be 1.01, and the suggested precision is 3:

var p = d3.precisionRound(0.01, 1.01),
    f = d3.format("." + p + "r");
f(0.99); // "0.990"
f(1.0);  // "1.00"
f(1.01); // "1.01"

Whereas for the numbers 0.9, 1.0, and 1.1, the step should be 0.1, the max should be 1.1, and the suggested precision is 2:

var p = d3.precisionRound(0.1, 1.1),
    f = d3.format("." + p + "r");
f(0.9); // "0.90"
f(1.0); // "1.0"
f(1.1); // "1.1"

Note: for the e format type, subtract one:

var p = Math.max(0, d3.precisionRound(0.01, 1.01) - 1),
    f = d3.format("." + p + "e");
f(0.01); // "1.00e-2"
f(1.01); // "1.01e+0"