Adding a new mark for creating vornoi diagrams

[josephburkhart]

Enhancement Description

I work in a materials science research group that is often interested in constructing phase diagrams from simulation results in multidimensional parameter-space. A common way for us to construct 2 (or 3) dimensional phase diagrams from points in parameter-space is by constructing a vornoi diagram from points that we have classified as belonging to specific phases. I would like to propose that we add a new vornoi mark that represents a collection of points in space as their vornoi diagram.

I'm somewhat new to collaborative open-source development, and I have never contributed to a Vega project, so please pardon any mistakes here. I think my proposal constitutes new syntax, so I will attempt to provide an example minimum spec below:

{
  "mark": {
    "type": "vornoi", 
    "filled": true
  },
  "encoding": {
    "x": {
      "field": "temperature",
      "type": "quantitative",
      "scale": {"zero": false}
    },
    "y": {
      "field": "pressure",
      "type": "quantitative",
      "scale": {"zero": false}
    },
    "color": {
      "field": "phase",
      "type": "nominal"
    }
  }
}

The full spec would look very similar to that of geoshape, except instead of the geometry being determined by projection of pre-defined coordinates, it would be calculated on the fly using a vornoi tessellation algorithm. In fact, there's already a discussion in the altair repo where someone uses the geoshape mark and scipy.spatial.Vornoi to provide a workaround for a vornoi diagram in an altair chart. The reason I'm proposing this enhancement here is that I would eventually like to see this functionality built into altair, so that people like me can use it to easily make interactive phase diagrams.

Here's what I'm not sure about:

• What would the mark look like if the user zoomed-out, so that the input points only occupied a small part of the canvas? Would the outer vornoi cells just appear to extend outward infinitely, or would we force the user to specify an outer boundary as a property?
• Would it be possible to re-use the existing implementation of the vornoi algorithm in vega-lite, where it is used to determine the nearest mark to the cursor?
• Would we want to add any additional encodings or properties to the vornoi mark? For example, I could imagine that perhaps some users might want to construct a weighted vornoi diagram, which would warrant a weight encoding and perhaps a weight-type property.

Checklist

• I checked for duplicate issues.