Improve label capacity calculation in time scale

[nagix]

Problem 1

Currently getLabelCapacity uses the displayFormats for 'millisecond' if unit is not specified. This results in less label capacity than actual (and this often happens). The example below shows that the unit is automatically set to 'month' when there are two labels ['2015-01-01', '2015-02-01'].

Problem 2

#6265 changed to compute the capacity using the major unit if applicable, but me._majorUnit is not set when getLabelCapacity is called. Furthermore, it is not always true that the label width of a major tick is larger than that of a minor tick or vise versa.

Solution

This PR takes into account the label capacity of both minor and major ticks in getLabelCapacity. And, determineUnitForAutoTicks checks the label capacity for each unit.

Potential problem of this change is that more labels will be rotated because getLabelCapacity calculates the capacity using rotation. But I think this should be controlled by the maxRotation option.

Master: https://jsfiddle.net/nagix/zdkcaerb/

This PR: https://jsfiddle.net/nagix/5kersouy/

Fixes #5093

[kurkle]

Looks good to me.

[nagix]

Rebased and reverted some changes for size reduction.

[benmccann]

@nagix I finished my other work on the time scale and auto-skipping. Thanks for letting me get those PRs in first. I really appreciate it. This one can be rebased now

[benmccann]

@nagix I'm going to close this since we haven't heard anything on it for awhile, but please feel free to reopen if you're still interested in pursuing