Change dataset from object to array of dataset objects

[shepazu]

We need the ability to have more than one dataset per chart, for a few use cases, including:

1. A chart may have multiple datasets, such as a line chart superimposed over a bar chart, like so:

2. A dashboard, especially one where two charts are closely related, may be rendered in a single chart component.

There are probably other use cases. In general, if there is more than one dataset and any of their facets aren't identical, it should be split into multiple dataset elements.

[jvartanian]

@shepazu Would it make sense to change the field name to be plural? e.g datasets

[shepazu]

That seems sensible. Semantically, it's common to use "dataset" to mean a collection of data tables, but by using the plural for the keyword, it would reinforce for the author that it can contain more than one "table" of data.

[shepazu]

Changed in most recent drafts.