[BUG] Null ticks are graphed when autoSkip=false

[elibyrd]

Expected Behavior

When a custom tick format callback returns null, that tick should be removed from graph.

Current Behavior

If autoSkip is set to false, null ticks are graphed alongside non-null ticks.

Possible Solution

It looks like null ticks are getting filtered out in _autoSkip (line 7492):

if (shouldSkip && i !== tickCount - 1 || helpers.isNullOrUndef(tick.label)) {
	// leave tick in place but make sure it's not displayed (#4635)
	delete tick.label;
}

If the null check were applied outside of the autoskip filter, it might fix the issue.

Steps to Reproduce (for bugs)

JSFiddle: https://jsfiddle.net/elibyrd/mxcervrt/

1. Create a custom tick format that returns null on at least one tick
2. Set autoSkip=false on same axis

Context

We have a dynamic chart that graphs one variable over the course of several hours, in minute increments. We want to manually set the x axis to tick every hour, which autoSkip interferes with.

Environment

• Chart.js version: 2.7.0
  Note: this wasn't an issue in version 2.6.0 - we encountered it when trying to upgrade to 2.7.0 to take advantage of some new features in 2.7.

[benmccann]

Thanks for reporting. I've sent a PR to fix this

[etimberg]

Fixed in #4795