Interacting with Legends & Axes

[brylie]

NVD3, and possibly other charting libraries, allow users to filter charts by interacting with the chart legend:

This is useful for exploratory analysis, and makes toggling categories very easy.

Feature

Is it planned/possible for vega-lite charts to have interactive legends for filtering chart data?

[kanitw]

Thanks for asking. This will likely be a part of v2.x release.

[domoritz]

Interactions with legends was a bit tricky with vega (at least v2). Has that changed? cc @arvind

[arvind]

Yes, with Vega 3, axes and legends can now be interacted with. So this should be supported with Vega-Lite 2.0. The open question with interactions, however, is what defaults should be. For example, if a legend is added to the visualization, should it be interactive by default? Should the interaction it supports also extend to interacting with the plotted data points as well? Etc.

[jheer]

We should definitely try to support these kinds of interaction! Not sure if interactive-by-default is the right way to go, but we would need to be able to provide explicit control in either case.

This relates to the selection project transform. As I understand it, our default design right now is no projection for single/multi point selections, and [x, y] projection for interval selections. For axes and continuous (color gradient) legends, I imagine one could define an interval selection projected to the appropriate field/channel/guide. For discrete legends, we might need to default a single-point or multi-point selection to project just the field(s) backing the legend.

There is also the potential issue that users might just want to be able to (more or less) say: "I want to enable selections on this legend (or axis)" without having to think about the appropriate selection type (interval, etc).

[domoritz]

I don't think we want any interactions by default. The problem is that I don't know what channel we would use (e.g. for highlighting) and how we resolve conflicts if the user uses that channel to encode data.

[kanitw]

RE: default interaction, let's create a new issue (#1658) to discuss that
since it's going a little off topic here.

[brylie]

For what it's worth, NVD3 chart legends are interactive by default.

[domoritz]

We have this on the roadmap for 2.1

[kanitw]

Given that we will follow semantic versioning, it is probably more accurate to say "very early versions of 2.x minor release".