Time Scale not calculating diffs correctly

[lenyy]

Hi there,
So there is a bug with the current calculation of dates in the buildLabelsDiff function of the time scale.
Currently at line 194 of scale.time.js where you have scaleLabelDiffs.push(labelMoment.diff(me.firstTick, me.tickUnit, true)); this will give you some other value different from 0 even if dates are equal, but not in all cases.
Lets imagine the scenario in this fiddle https://jsfiddle.net/lenyy/scgc7tcj/ , i create two date objects but they have one hour difference between them, so calculating the diff between them using days has unit should give zero, it does if you don't pass true has the third argument to diff function that says to diff function that you wanna use floor. Moment changed the way the used to round values https://gist.github.com/timrwood/e72f2eef320ed9e37c51#momentdiff-now-floors-instead-of-rounds
they now floor instead of round.

So the fix is to change scaleLabelDiffs.push(labelMoment.diff(me.firstTick, me.tickUnit)); without the last argument, so now when your value falls on the same value of a tick it will return 0.

I think this has lead to several other problems that i have saw in here when using bar chart that values we're shiffted and i think this might be the case when using day as your tick unit. i haven't tested for other time units, but it should be the same.

Any questions just ask :)

Cheers,
Pedro

[etimberg]

@lenyy i think you're probably right about the cause of the bug. Ultimately it might be simpler to rewrite the time scale to be based on a linear scale that formats as time values. I prototyped that in https://github.com/chartjs/Chart.js/tree/time-scale-improvements

[lenyy]

@etimberg i was somewhat right and wrong at the same time lol. So the way you calculate the diffs is correct only if your date doesn't fall in the tickMoments array, if your date is one from the tickMoments then you should make labelMoment.diff(me.firstTick, me.tickUnit, true) because if we are not a tick we want to get the float value that is closer to us. If you check #3574 you can see in my fiddle where i have both cases covered in the buildLabelDiffs function, but that case is very specific since i'm building a timeLine chart.

I've only read your improvements but haven't tested, but it makes sense to use linear scale with values converted from unix timestamp to moment dates. I'll try it after.

[etimberg]

I believe this was fixed in #3914