d3.svg.axis().ticks(d3.time.day, n) sets ticks at end and beginning of certain months for certain intervals

I’m not sure there’s much that can be done about this, but for certain intervals you’ll get ticks at both the end and the front of the certain months.

For something like d3.svg.axis().ticks(d3.time.day, 3 /* or 1, 5, 10... anything divisible by 30 */), the ticks will look something like this:

Jan 25   Jan 28   **Jan 31   Feb 1**   Feb 3

This is because the conditional evaluates to true: https://github.com/mbostock/d3/blob/a40a611d6b9fc4ff3815ca830d86b6c00d130995/d3.js#L2414 on the first of the month and end of the month.

Following the D3 logic, here’s an example for axis.ticks(d3.time.day, 3):

// True for January 31
var dt = 3;
var d = new Date('January 31')
var time = d.getDate() - 1; // 30
var m = time % dt // 30 % 3 == 0
if (!m) times.push(d);

// True for February 1
var d = new Date('February 1')
var time = d.getDate() - 1; // 0
var m = time % dt // 0 % 3 == 0
if (!m) times.push(d);

[KamilSzot]

Isn’t there any way to place a tick every 3 days regardless of the month? Something like every 3 total days not every 3rd day of the month?

[mbostock]

Careful: that implementation appears to have a bug in time zones that have daylight saving time changes at midnight, or for dates before epoch (January 1, 1970)… but… close enough for the vast majority of use cases.

I will consider this issue for the new 4.0 API in d3-time and d3-scale. I’ve created two related issues, d3/d3-time#6 and d3/d3-scale#7. It’s pretty easy to create a three-day interval (regardless of month) in the new API thanks to interval.filter and day.count:

var threeday = day.filter(function(d) { return day.count(0, d) % 3 === 0; });

But I haven’t yet exposed a way to use such a custom interval with the new time scale. I’ll probably just allow you to pass an interval—or possibly a range function—to time.ticks, as before. (Though, I don’t think the new time scale will allow you to specify the optional step if you pass in a custom interval.)