[Accessibility] Tooltip content is not accessible to screen readers. about vega

[domoritz]

The current behavior somewhat by design. Tooltips are designed for mouse-interactions. The details of the marks should be added to the marks as aria properties so that the tooltips are not needed at all. See https://vega.github.io/vega/docs/marks/#accessibility-properties-51. Vega-Lite does that by default already IIRC.

However, I see that that is not aways the case and that we could do a better job with documentation and maybe exposing the tooltip values by default to a screen reader.

I don't know how to act on this current issue since Vega is fairly low-level and so it's more up to the developer how they want to use the library. We could definitely improve docs and also the default behavior in Vega-Lite but those would need separate and more specific issues.