"hasGroupedData" with multiple series does not return the right status

[ernstfabian]

Expected behaviour

Returning "true" or "undefined" when calling "hasGroupedData" on a series, depending if the data is currently beeing grouped.

Actual behaviour

When two or more series are displayed, it always returns "true" once it returned "true" at least once.
When there is only one series inside the graph it works as expected - returns "true" when the data is beeing grouped and "undefined" when zoomed in far enough where grouping is not applied. Also returns back to "true" when you reset the zoom and back to "undefined" when zoomed in again.

Live demo with steps to reproduce

Stackoverflow
https://jsfiddle.net/Itsearnest/ho0a4zw9/

Product version

Highstock v10.0.0

Affected browser(s)

Tested on Firefox, Chrome and Edge

[karolkolodziej]

Works well in version 9.1.2: https://jsfiddle.net/BlackLabel/jfg1zh8a/

Internal note:
Bisected to commit 2f23d6a

The getGroupPixelWidth in the applyGrouping method returns a value greater than zero when it shouldn't.
Then based on that knowledge the grouping is being applied.

Before the getGroupPixelWidth the groupPixelWidth is reset but only for the current series.
Because of that getGroupPixelWidth falsely take the groupPixelWidth from the previously calculated series.
Where previously means previous extremes and different arrangement of points unrelated to the current state of chart and series.

highcharts/ts/Extensions/DataGrouping.ts

Lines 1173 to 1182 in 53b04dc

	if (
	series[i].groupPixelWidth ||
	(
	dataLength >
	((this.chart.plotSizeX as any) / groupPixelWidth)
	) ||
	(dataLength && dgOptions.forced)
	) {
	doGrouping = true;
	}

The suggested fix is to apply code from the snippet below before the group pixel calculation.

series.forEach(function(series) {
   series.groupPixelWidth = void 0;
})