step2.js

svg.attr('width', width);
svg.attr('height', height);

const margin = {
  top    : 20,
  right  : 80,
  bottom : 30,
  left   : 30
};

const chartWidth     = +svg.attr("width") - margin.left - margin.right;
const chartHeight    = +svg.attr("height") - margin.top - margin.bottom;
const chartContainer = svg.append("g").attr("transform", "translate(" + margin.left + "," + margin.top + ")");