Logarithmic (“log”) scales are like linear scales except that a logarithmic transform is applied to the input domain value before the output range value is computed. The mapping to the range value y can be expressed as a function of the domain value x: y = m log(x) + b.
As log(0) = -∞, a log scale domain must be strictly-positive or strictly-negative; the domain must not include or cross zero. A log scale with a positive domain has a well-defined behavior for positive values, and a log scale with a negative domain has a well-defined behavior for negative values. (For a negative domain, input and output values are implicitly multiplied by -1.) The behavior of the scale is undefined if you pass a negative value to a log scale with a positive domain or vice versa.
const x = d3.scaleLog([1, 10], [0, 960]);
If domain is not specified, it defaults to [1, 10]. If range is not specified, it defaults to [0, 1].
const x = d3.scaleLog([1, 1024], [0, 960]).base(2);
If base is not specified, returns the current base, which defaults to 10. Note that due to the nature of a logarithmic transform, the base does not affect the encoding of the scale; it only affects which ticks are chosen.
const x = d3.scaleLog([1, 100], [0, 960]); const T = x.ticks(); // [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100]
If the base is an integer, the returned ticks are uniformly spaced within each integer power of base; otherwise, one tick per power of base is returned. The returned ticks are guaranteed to be within the extent of the domain. If the orders of magnitude in the domain is greater than count, then at most one tick per power is returned. Otherwise, the tick values are unfiltered, but note that you can use log.tickFormat to filter the display of tick labels. If count is not specified, it defaults to 10.
const x = d3.scaleLog([1, 100], [0, 960]); const T = x.ticks(); // [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, …] const f = x.tickFormat(); T.map(f); // ["1", "2", "3", "4", "5", "", "", "", "", "10", …]
If there are too many ticks, the formatter may return the empty string for some of the tick labels; however, note that the ticks are still shown to convey the logarithmic transform accurately. To disable filtering, specify a count of Infinity.
When specifying a count, you may also provide a format specifier or format function. For example, to get a tick formatter that will display 20 ticks of a currency, say
log.tickFormat(20, "$,f"). If the specifier does not have a defined precision, the precision will be set automatically by the scale, returning the appropriate format. This provides a convenient way of specifying a format whose precision will be automatically set by the scale.
const x = d3.scaleLog([0.201479, 0.996679], [0, 960]).nice(); x.domain(); // [0.1, 1]
If the domain has more than two values, nicing the domain only affects the first and last value. Nicing a scale only modifies the current domain; it does not automatically nice domains that are subsequently set using log.domain. You must re-nice the scale after setting the new domain, if desired.