period positioning

[nicolaskruchten]

This issue is intended to apply in principle to scatter (not scattergl), bar, funnel, waterfall, ohlc and candlestick traces, and only applies in the context of (x|y)axis.type=date.

We need to be able to position marks along date axes not at a particular instant in time, but at the start/midpoint/end of a well-defined period (see related #4911).

Proposed new trace-level attributes:

• (x|y)period: Standard duration notation like ‘m’ or millis
• (x|y)period0: A specific boundary point timestamp which is used to compute all boundaries. This will allow us to define e.g. "quarters that start on Feb 1" or "weeks that start on Wednesdays" etc
• (x|y)periodalignment: start/middle/end

Proposed backwards-compatible changes to traces with width parameter:

• Start accepting period lengths
• Defaults to (x|y)period if orientation=(v|h)
• If specified as a period, then the bar widths are variable and the gaps are computed as a percentage of the local bar width

Note: the hoverlabel here will need some thought.

[nicolaskruchten]

Re the hover-label, let's not do anything special for now: the user can do whatever they like in the hovertemplate.

Re the xperiod0 concept, I'll note that setting the default value to 1950-01-01 00:00 conveniently anchors seconds, minutes, hours at 0, the day of week on a Sunday, the 1st of the month at 1, the first month of the year at January, the first quarter at Jan/Feb/Mar and the first year of a decade at a multiple of 10.

If someone wants quarters like Q1=Feb/Mar/Apr then could set xperiod0 to something like 2020-02-01 or any other February 1, and if they want weeks that start on Mondays then they could set xperiod0 to something like 2020-08-31 or any other Monday

[alexcjohnson]

FYI the default tick0 behavior already shifts to Sunday when using week ticks:

plotly.js/src/lib/dates.js

Lines 45 to 58 in 2737bf2

	/*
	* dateTick0: get the canonical tick for this calendar
	*
	* bool sunday is for week ticks, shift it to a Sunday.
	*/
	exports.dateTick0 = function(calendar, sunday) {
	if(isWorldCalendar(calendar)) {
	return sunday ?
	Registry.getComponentMethod('calendars', 'CANONICAL_SUNDAY')[calendar] :
	Registry.getComponentMethod('calendars', 'CANONICAL_TICK')[calendar];
	} else {
	return sunday ? '2000-01-02' : '2000-01-01';
	}
	};

[nicolaskruchten]

Given Alex's comment above, let's just do the same thing with xperiod0 then, no need to force it to 1950 :)