Inject empty data at top level for easier datum encodings

[jakevdp]

.cc @mattijn

I think this is roughly what you had in mind in the previous discussion (#2513 (comment)). We may want a more sophisticated version of this rather than always adding empty data at the top level.

[ChristopherDavisUCI]

I saw in the Vega-Lite schema that "type": "null" is an allowable option for the data property. Is there a standard way to use that in Altair? For me writing something like alt.Chart(None) is just as nice (or maybe even nicer since it's more explicit) as writing alt.Chart() if we want to specify empty data.

[jakevdp]

Interesting - where do you see that {"type": null} is a valid option for data? When I look at the v4 schema, I only see URLData (which requires "url"), NamedData (which requires "name"), and InlineData (which requires "values"). Are you talking about the v5 schema?

[ChristopherDavisUCI]

Maybe you're one level deeper in the schema than I am? I see "type": "null" under:
TopLevelUnitSpec->properties->data.

[jakevdp]

Ah, yeah I see. I think the meaning of data=None is different than what we want here, though:

"description": "An object describing the data source. Set to `null` to ignore the parent's data source. If no data is set, it is derived from the parent."

So data=Undefined (the default in alt.Chart()) means to use the parent's data, if any, and data=None (alt.Chart(None)) means ignore parent data and act as if there's no data set for this subchart.

For showing datum encodings, we actually do want data to be defined, but we want a single row with no columns, i.e. data={'values': [{}]}.

[ChristopherDavisUCI]

I see! I often find those schema descriptions difficult to parse, so I've often gotten in the habit of not even looking at them.

If there's anything you'd like me to try related to this empty encoding, just let me know (I couldn't tell if there was more to be done or not). But for now I'll go back to thinking about version 5.

[mattijn]

I played a bit with this; I think it works good.