var spec = {
    "data": {
      "values": [
        {"a": "C", "b": 2}, {"a": "C", "b": 7}, {"a": "C", "b": 4},
        {"a": "D", "b": 1}, {"a": "D", "b": 2}, {"a": "D", "b": 6},
        {"a": "E", "b": 8}, {"a": "E", "b": 4}, {"a": "E", "b": 7}
      ]
    },
    "mark": "bar",
    "encoding": {
      "y": {"field": "a", "type": "nominal"},
      "x": {
        "aggregate": "average", "field": "b", "type": "quantitative",
        "axis": {
          "title": "Average of b"
        }
      }
    }
  };

var opt = {
  "mode": "vega-lite"
};

vega.embed("#vis", spec, opt, function(error, result) {
  // Callback receiving the View instance and parsed Vega spec
  // result.view is the View, which resides under the '#vis' element
});