locale.formatPrefix()

locale.formatPrefix(specifier, value)

Equivalent to locale.format, except the returned function will convert values to the units of the appropriate SI prefix for the specified numeric reference value before formatting in fixed point notation. The following prefixes are supported:

• y - yocto, 10⁻²⁴
• z - zepto, 10⁻²¹
• a - atto, 10⁻¹⁸
• f - femto, 10⁻¹⁵
• p - pico, 10⁻¹²
• n - nano, 10⁻⁹
• µ - micro, 10⁻⁶
• m - milli, 10⁻³
• ​ (none) - 10⁰
• k - kilo, 10³
• M - mega, 10⁶
• G - giga, 10⁹
• T - tera, 10¹²
• P - peta, 10¹⁵
• E - exa, 10¹⁸
• Z - zetta, 10²¹
• Y - yotta, 10²⁴

Unlike locale.format with the s format type, this method returns a formatter with a consistent SI prefix, rather than computing the prefix dynamically for each number. In addition, the precision for the given specifier represents the number of digits past the decimal point (as with f fixed point notation), not the number of significant digits. For example:

var f = d3.formatPrefix(",.0", 1e-6);
f(0.00042); // "420µ"
f(0.0042); // "4,200µ"

This method is useful when formatting multiple numbers in the same units for easy comparison. See precisionPrefix for help picking an appropriate precision, and bl.ocks.org/9764126 for an example.