Vega-Lite: rule line over heatmap

[jak...@cs.washington.edu]

Hello,

I've been playing with the development version of Vega-Lite, particularly the layered charts. One thing I could not figure out how to do is add a rule line to a heatmap.  For example, what layer should I create to add a vertical rule to this plot?

{

  "data": {

    "url": "https://vega.github.io/vega-datasets/data/cars.json"

  },

  "mark": "text",

  "encoding": {

    "text": {"value": " "},

    "row": {"field": "Origin"},

    "column": {"field": "Cylinders"},

    "color": {"aggregate": "average","field": "Miles_per_Gallon"}

  },

  "config": {"mark": {"applyColorToBackground": true}}

}

Any encoding I've tried with {... "mark": "rule"  ...} as a layer results in an error.

FYI – the reason for this is I want to be able to complete this example: http://nbviewer.jupyter.org/github/ellisonbg/altair/blob/master/altair/notebooks/12-Measles.ipynb

Thanks!

   Jake