Implement per-dataset type (default and per-chart) options

[benmccann]

This fixes mixed line and scatter charts. Issue described in #4587

Closes #5997 & #5151

[kurkle]

After the discussion in Slack, this seems like a good start!

[kurkle]

Looks good to me

[simonbrunel]

A minor update in _defaults.

We also need unit tests per controller (line + scatter) to make sure that dataset defaults are correctly handled for every options. And I guess we also need to update the docs about configuring dataset defaults.

[benmccann]

We also need unit tests per controller (line + scatter) to make sure that dataset defaults are correctly handled for every options.

The line chart showLine option was already well tested. I added additional tests for spanGaps and for scatter's showLine.

And I guess we also need to update the docs about configuring dataset defaults.

We don't allow users to configure default options. defaults._set is private

[kurkle]

One line to remove and a question to consider.

[simonbrunel]

We don't allow users to configure default options. defaults._set is private

_set is indeed an internal helper but users are allowed to modify Chart.defaults.datasets.*. So since we are introducing a new public API that allows users to configure defaults for all dataset options, we should also add unit tests for it.

[benmccann]

Ok, I went ahead and added documentation. I had already added unit tests

[MatsNissen-Lie]

I want to combined the candlestick chart with a line chart. Is that possible, and if so, how?