Add Gapminder Tracking

[thinkh]

Closes #11

Usage

Tracking is implemented by adding a track property to a signal definition.

{
  "name": "colorField",
  "value": "continent",
  "bind": {"input": "select", "options": ["continent", "main_religion"]},
  "track": {
	"title": "Color = {{value}}"
  }
}

The title property is used for the node name in the provenance graph and can contain Handlebar.js syntax.

In case further data is required to generate the title we have to wait until Vega's dataflow evaluation is complete. Further values from other data or signals can then be acquired by using the async property.

{
  "name": "selectedCountries",
  "on": [
	{"events": {"signal": "clicked"}, "update": "null", "force": true}
  ],
  "track": {
	"async": [
	  {"data": "selectedCountries", "as": "allCountries"},
	  {"signal": "clicked", "as": "lastCountry"}
	],
	"title": "Selected {{lastCountry.country}} ({{allCountries.length}} Countries)"
  }
}

It is also possible to define a category and an operation for the node in the provenance graph:

...
  "track": {
	"title": "Selected {{lastCountry.country}} ({{allCountries.length}} Countries)",
    "category": "selection",
    "operation": "update"
  }
...

Possible options are:

• category: data (default), selection, visual, layout, logic, custom, annotation
• operation: create, update (default), remove

Open Issues

• Find better way to track numerical signals #13