From mediawiki.org


  "version": 2,
  "width": 300,
  "height": 300,
  "data": [
      // Data is retrieved from the MediaWiki API.  By default it uses current wiki's "Extension by category"
      "name": "table",
      "url": "wikiapi:///?generator=categorymembers&gcmtitle=Category:Extensions+by+category&gcmtype=subcat&action=query&gcmlimit=max&prop=categoryinfo&formatversion=2&format=json",
      // We are only interested in the content of query.pages subelement.
      "format": {"property": "query.pages","type": "json"},
      "transform": [
        // sort in descending order using category size as the sort key
        {"type": "sort","by": "-categoryinfo.size"},
        // To visualize, use "pie" transformation to add layout_start, layout_end, and layout_mid fields to each page object
        // These fields contain angles at which to start and stop drawing arcs. First element's start will be 0, and last element's end will be 360 degrees (in radians)
        {"type": "pie","field": "categoryinfo.size"}
  // Scales are like functions -- marks use them to convert a data value into a visual value, like x or y coordinate on the graph, or a color value.
  "scales": [
      // This scale will be used to assign a color to each slice, using a palette of 10 colors
      "name": "color",
      "domain": {"data": "table","field": "title"},
      "range": "category10",
      "type": "ordinal"
  "marks": [
      // This mark draws the actual pie chart from the data source
      // Each element is an arc between layout_start and layout_end angles (as calculated by the pie transformation)
      // drawn with a given radius, stroke, and fill.
      "from": {"data": "table"},
      "type": "arc",
      "properties": {
        "enter": {
          "fill": {"scale": "color","field": "title"},
          "outerRadius": {"value": 200},
          "startAngle": {"field": "layout_start"},
          "endAngle": {"field": "layout_end"},
          "stroke": {"value": "white"},
          "strokeWidth": {"value": 1}
      // This mark draws labels around the pie chart after the pie chart has been drawn
      "type": "text",
      // Before drawing, we need to perform a number of calculations to figure out the exact location and orientation of the text
      "from": {
        "data": "table",
        "transform": [
          // For each data point (datum), each of these transformations will be ran in order.
          // Formula transformation evaluates the expression and assigns result to the datapoint
          // Size of the pie slice, in degrees:  sliceSize = (end - start) * 180 / Pi
          { "type": "formula", "field": "sliceSize", "expr": "(datum.layout_end - datum.layout_start)*180/PI" },
          // Draw text only if the slice of the arc is more than 2 degrees to avoid overcrowding
          { "type": "filter", "test": "datum.sliceSize > 2" },
          // Remove namespace from the text - keeps only text after the first ':' symbol, limits to 40 chars.
          { "type": "formula", "field": "title", "expr": "substring(datum.title, 1+indexof(datum.title,':'), 40)" },
          // Determine the side of the pie chart we are on - left or right.
          { "type": "formula", "field": "invert", "expr": "datum.layout_mid*180/PI < 180 ? 1 : -1" },
          // If on the left, the text should be right-aligned (go from the rim inward)
          { "type": "formula", "field": "align", "expr": "datum.invert < 0 ? 'left' : 'right'" },
          // At what angle should the text be drawn relative to the point on the circle
          { "type": "formula", "field": "angle", "expr": "(datum.layout_mid*180/PI)-90*datum.invert" },
          // Make font smaller for smaller pie slices
          { "type": "formula", "field": "fontSize", "expr": "datum.sliceSize > 20 ? 15 : (datum.sliceSize > 10 ? 14 : 10)" },
          // Make font bold for largest pie slices 
          { "type": "formula", "field": "fontWeight", "expr": "datum.sliceSize > 15 ? 'bold' : 'normal'" }
      "properties": {
        "enter": {
          // Use the fields calculated in the transformation to draw category names
          "align": {"field": "align"},
          "angle": {"field": "angle"},
          "baseline": {"value": "middle"},
          "fill": {"value": "black"},
          "fontSize": {"field": "fontSize"},
          "fontWeight": {"field": "fontWeight"},
          "radius": {"value": 270},
          "text": {"field": "title"},
          "theta": {"field": "layout_mid"}