var places = {
  NY: [ -74.007124, 40.71455 ],
  LA: [ -118.245323, 34.05349 ]
};

var route = {
  type: "LineString",
  coordinates: [
    places.NY,
    places.LA
  ]
};

layerElement.append("path")
  .datum(route)
  .attr("class", "route")
  .attr("d", path)
  .style('fill', 'none')
  .style('stroke', 'red')
  .style('stroke-width', '3px');