overlayGeoJson (json, name, keyAccessor) {
        for (let i = 0; i < this._geoJsons.length; ++i) {
            if (this._geoJsons[i].name === name) {
                this._geoJsons[i].data = json;
                this._geoJsons[i].keyAccessor = keyAccessor;
                return this;
            }
        }
        this._geoJsons.push({name: name, data: json, keyAccessor: keyAccessor});
        return this;
    }
**mandatory**

Use this function to insert a new GeoJson map layer. This function can be invoked multiple times
if you have multiple GeoJson data layers to render on top of each other. If you overlay multiple
layers with the same name the new overlay will override the existing one.
@see {@link http://geojson.org/ GeoJSON}
@see {@link https://github.com/topojson/topojson/wiki TopoJSON}
@see {@link https://github.com/topojson/topojson-1.x-api-reference/blob/master/API-Reference.md#wiki-feature topojson.feature}
@example
// insert a layer for rendering US states
chart.overlayGeoJson(statesJson.features, 'state', function(d) {
     return d.properties.name;
});
@param {_geoJson} json - a geojson feed
@param {String} name - name of the layer
@param {Function} keyAccessor - accessor function used to extract 'key' from the GeoJson data. The key extracted by
this function should match the keys returned by the crossfilter groups.
@returns {GeoChoroplethChart}