The decimal format (d) isn’t observed if x ≥ 1e21.

[mbostock]

For example:

var f = d3.format(",d");
f(1e20); // "100,000,000,000,000,000,000"
f(1e21); // "1e+21"

Because:

(1e20).toString(10); // "100000000000000000000"
(1e21).toString(10); // "1e+21"

Related #21.

[mbostock]

On the other hand, toLocaleString seems to do what we want:

(1e21).toLocaleString("en") // "1,000,000,000,000,000,000,000"

[mbostock]

Fixed in #104.