"config": {"axisX": {"title": "foo"}} removes title

[mattijn]

As was raised at the Altair repo: vega/altair#3582

When defining the x-axis title using the general config object, as such: (Open the Chart in the Vega Editor)

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.20.1.json",
  "data": {"values": [{"a": 1, "b": 4}, {"a": 2, "b": 5}, {"a": 3, "b": 6}]},
  "mark": {"type": "line"},
  "encoding": {"x": {"field": "a"}, "y": {"field": "b"}},
  "config": {"axisX": {"title": "foo"}}
}

It is rendered like:

As can be seen, the x-axis title is removed completely.

Upon inspecting the compilation process, I noticed that the general axis configuration in Vega-Lite is directly mapped to a Vega config object. But, the Vega config object is essentially a subset of the available Vega-Lite configuration for axis properties. Since title is not defined as one of the axis properties in the Vega general config (source), but it is clearly defined as a Vega-Lite axis property, leading to a mismatch.

I believe there are a couple of possible solutions:

• The VL schema could be adjusted to reflect that the general axis config object in Vega-Lite is a subset of the available axis properties.
• The compilation process from Vega-Lite to Vega could be improved or extended to ensure it does not directly map the general config object to Vega but also accounts for Vega-Lite-specific properties like title.

It's odd though, that Vega removes the axis title during rendering, even though it's not part of its schema.

[clairenied]

Hey! Is this a good first issue? If so, I might be interested in taking it. Let me know!

[domoritz]

The config is for general properties and I think the specific axis title shouldn't be supported (which is how Vega behaves). We should not allow it in Vega-Lite and remove support for it from the schema (via removing it from the allowed types). @clairenied, yes, this is a great issue to tackle.

[clairenied]

Sweet! I will start poking around on this.

[clairenied]

Oh! It looks like I am not authorized to assign the issue to myself, so if you would like, please feel free to assign it to me if that is in line with your workflow.

[domoritz]

No need to assign the issue formally from our side.