a D3 version 4 fork of @micahstubbs' bl.ock ES2015 X-Value Mouseover with Droplines
D3 v4 syntax cues taken from mbostock's bl.ock Line Chart 4.0
https://d3js.org/d3.v4.min.js
https://d3js.org/d3-selection-multi.v0.4.min.js
https://npmcdn.com/babel-core@5.8.34/browser.min.js