Bugged rounding algorithm when setting Extremes for y axis

[ThomasK0lasa]

Expected behaviour

Better rounding: 106 rounded to 120;

Actual behaviour

Wrong rounding 106 rounded to 200. This happens for ONLY specific min and max. Take a look at examples below.

Live demo with steps to reproduce

Proper rounding: 99 rounded to 100 > https://jsfiddle.net/y9cz1Lu0/4/
Proper rounding: 112 rounded to 120 > https://jsfiddle.net/y9cz1Lu0/5/
WRONG rounding: 106 rounded to 200 (BUGGED) > https://jsfiddle.net/2mn8yd0k/2/

As You can see the rounding for specific min max is wrongly calculated (106 rounded to 200?). Please don't tell me that I can do endOnTick: false or change the tickAmount as this is not solving the problem, but merely omitting it.

Product version

Highcharts JS v10.0.0 (2022-03-07)

Affected browser(s)

Chrome 99.0.4844.51

[karolkolodziej]

In steps how this mechanism works for alleged bugged demo:

1. In the setTickInterval method we calculate tickInterval = 25
2. Then in the setTickPositions we are trying to calculate where on the axis ticks should be.
   This results in tickPositions = [0, 25, 50, 75, 100, 125]
3. As you can see, the amount of ticks is greater than what you set by tickAmount: 5 that is why the adjustTickAmount has to be invoked.
4. There the tick interval is incremented like this axis.tickInterval *= 2 and setTickPositions fired once again.
5. This time we get tickPositions = [0, 50, 100, 150].
6. Now tick amount is smaller than the one set in the options.
   That is why the algorithm adds another tick and we get [0, 50, 100, 150, 200]

This mechanism is sensitive to the data and axis options and results might differ in case of a small change.
@TorsteinHonsi do you see any room for improvement?

[TorsteinHonsi]

Thanks for debugging @karolkolodziej! Yes it is true that this approach is sensitive to small variations in the data extremes. It seems like this is an unfortunate edge case where neither step 2 nor step 5 hit the mark, so we eventually end up with too much padding.

The only thing I can think of if we were able to modify those steps to hit the desired tickAmount right away, so we didn't have to modify it later. Feel free to experiment with it.

[TorsteinHonsi]

I'm trying a more granular increment instead of the *2 in step 4. Pushing a POC soon, let's see what visual differences the CI finds.

[karolkolodziej]

@ThomasK0lasa your chart with the new algorithm: https://jsfiddle.net/BlackLabel/vxn7j9L4/

[ThomasK0lasa]

@karolkolodziej thank you. Looks great. When this fix will be available in production? More or less.

[karolkolodziej]

The next release is planned for the following month.