Fix wrong ticks distribution for timescale

[Andgoncharov]

Fix wrong ticks distribution when time.min and/or time.max are beyond the visible part of a chart.

Code to reproduce:
var options = myChart.scales['timescale'].options;
options.time.min = start;
options.time.max = end;
myChart.update(0);

[benmccann]

@simonbrunel looks like this was last touched as part of #4582, so you probably know the logic best

[simonbrunel]

Reported issue #4845, need a test case.

[simonbrunel]

I'm removing the milestone since we don't know exactly what's the bug is (if it's really a bug).

[simonbrunel]

I think that's wrong, if min !== MAX_INTEGER, we still want Math.min(min, timestamps[0]); because min can change line 545 to a lower value. I don't understand why these 2 lines need to be modified, @Andgoncharov can you elaborate why these changes?

[etimberg]

I agree about elaborating this. @Andgoncharov could you post images with and without these changes? I also agree that we need to keep the Math.min() call.

[Andgoncharov]

Hi @simonbrunel, @etimberg,

I've created a fiddle https://jsfiddle.net/andgoncharov/8bgen7b7/ to display the problem. When I change options.time.min and options.time.max values, I expect that chart is displayed within specified borders and ticks are also distributed between those values. Currently, min and max values ain't taken into account if they are inside (timestamps[0], timestamps[length - 1]) interval.

When I apply my changes, the updated chart contains a number of x-axis labels according to the settings I've specified upon creation of the chart. Or may be I misunderstood something?

[etimberg]

I agree about elaborating this. @Andgoncharov could you post images with and without these changes? I also agree that we need to keep the Math.min() call.

[simonbrunel]

@Andgoncharov: If I understand correctly, when ticks.source: 'auto', you would like ticks to be generated based on the effective visualized range instead of the actual data range, meaning that the computed units and/or step size would change if the options min/max are far from the data min/max. I "think" you right, what do you think @etimberg @benmccann?

In this case, I think the proper fix is to ensure min/max options at line 554:

// Enforce limits with user min/max options
min = parse(timeOpts.min, me) || min;
max = parse(timeOpts.max, me) || max;

Then I think we can change min/max initialization (line 507) for:

var min = MAX_INTEGER;
var max = MIN_INTEGER;

Does it work?

Unit tests would certainly need to be changed since right now we don't expect time.min/max to impact the label generation.

[benmccann]

Yes, @simonbrunel I agree with your suggested fix

[Andgoncharov]

@simonbrunel, you've formulated it very clear. I did the change you've proposed and it looks correct for me. Thank you.

[simonbrunel]

Fixed by #4860