Support axis labelHref

[SyntaxRules]

Purpose:
Enable href navigation when user clicks on axis label

Explanation:
vega/vega-lite already has some form of interactivity. Interaction is mostly based on point selection. This feature proposes adding in interactivity to the axis labels. While my immediate desire is to enable href on axis', conditional highlighting or label selection might be nice as well.

Possible Spec:

{
  "$schema": "https://vega.github.io/schema/vega-lite/v2.json",
  "description": "A simple bar chart with ranged data (aka Gantt Chart).",
  "data": {
    "values": [
      {"task": "A","task_href": "google.com/A","start": 1, "end": 3},
      {"task": "A","task_href": "google.com/A","start": 4, "end": 7},
      {"task": "B","task_href": "google.com/B","start": 3, "end": 8},
      {"task": "C","task_href": "google.com/C","start": 8, "end": 10},
      {"task": "C","task_href": "google.com/C","start": 1, "end": 2}
    ]
  },
  "mark": "bar",
  "encoding": {
    "y": {"field": "task", "type": "ordinal", "axis": {"href": "task_href"}},
    "x": {"field": "start", "type": "quantitative"},
    "x2": {"field": "end", "type": "quantitative"}
  }
}

[domoritz]

Related to #1657

[willcrichton]

Is there a way to get this behavior with just Vega right now? I tried something like:

<VegaLite ... onNewView={v => v.addEventListener('@legendLabel:click', ...)} />

However, according to vega/vega#352 you can only listen to primitive events like click. I tried that too, but because legend elements aren't datum, they aren't passed to the click callback.

Is there maybe some indirect way where I can define a signal and listen to the signal? I tried this too:

<VegaLite 
   ...
   patch={spec => {
     spec.signals.push({
       name: "foobar",
       value: null,
       on: [{events: "@legendSymbol:click @legendLabel:click",
               update: '{value: datum.value}',
               force: true}]
     })
     return spec;
   }}
   signalListeners={{
     "foobar": (e) => {
       console.log('test', e);
     }
   }}
  />

But no luck either.

[kanitw]

@domoritz It's probably fair to add labelHref?

[domoritz]

I wonder whether it would be better to support labelHref in Vega directly rather than only Vega-Lite.

[flip111]

would be a nice feature :)