bar chart from json array

<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://d3js.org/d3.v4.min.js"></script>
  <style>
    body { margin:0;position:fixed;top:0;right:0;bottom:0;left:0; }
    
    #card_analytics__requests_by_period {
      max-width: 100%;
      background: #f7f7f7;
      height: 300px;
      display: flex;
      justify-content: center;
      align-items: center;
      overflow-x: auto;
    }
  </style>
</head>

<body>
  <div id="card_analytics__requests_by_period"></div>
  <script>
    
    // DATA
    
    var data = [
    {
      data_month: "2017-01-31",
      label: "January",
      requests_submitted: {          
        current: 21,
        prior: 16,
        during:236
      }
    }, {
      data_month: "2017-02-28",
      label: "February",
      requests_submitted: {          
        current: 16,
        prior: 43
      }
    }, {
      data_month: "2017-03-31",
      label: "March",
      requests_submitted: {          
        current: 36,
        prior: 20
      }
    }, {
      data_month: "2017-04-30",
      label: "April",
      requests_submitted: {          
        current: 230,
        prior: 310
      }
    }, {
      data_month: "2017-05-31",
      label: "May",
      requests_submitted: {          
        current: 50,
        prior: 14
      }
    }, {
      data_month: "2017-06-30",
      label: "June",
      requests_submitted: {          
        current: 39,
        prior: 47
      }
    }, {
      data_month: "2017-07-31",
      label: "July",
      requests_submitted: {          
        current: 72,
        prior: 68
      }
    }, {
      data_month: "2017-08-31",
      label: "August",
      requests_submitted: {          
        current: 31,
        prior: 97
      }
    }, {
      data_month: "2017-09-30",
      label: "September",
      requests_submitted: {          
        current: 57,
        prior: 25
      }
    }, {
      data_month: "2017-10-31",
      label: "October",
      requests_submitted: {          
        current: 71,
        prior: 45
      }
    }, {
      data_month: "2017-11-30",
      label: "November",
      requests_submitted: {          
        current: 25,
        prior: 32
      }
    }, {
      data_month: "2017-12-31",
      label: "December",
      requests_submitted: {          
        current: 41,
        prior: 21
      }
    }]

// VARS
    
    /* margin object for parent container */
    var margin = {top: 20, right: 20, bottom: 20, left: 40};
    
    /* parent container id */
    var container = '#card_analytics__requests_by_period'
    
    /* width of parent container that will by dynamic in ReloQuest app. For now it is static */
    var parentWidth = 900
    var parentHeight = 300
    
    /* width and height of the chart area. This is defined so that margins aren't needed in later calculations */
    var width = parentWidth - margin.left - margin.right
    var height = parentHeight - margin.top - margin.bottom
    
    /* Will be used to separate each month's data into current and prior periods */
    var keys = Object.keys(data[0].requests_submitted)
    
    
    
    // SVG
    
    /* the svg element's width and height is identical to the parent width and height. Instead of just using the `width`
       and `height` variables declared earlier, we are basically setting some padding on the entire svg element so calculations
       don't need to take it into account */
    
    var svg = d3.select(container).append('svg')
    	.attr('width', width + margin.left + margin.right)
    	.attr('height', height + margin.top + margin.bottom)

/* The g element will be where the chart actually lives */
  	var g =	svg.append('g')
    		.attr('transform', 'translate(' + margin.left + ',' + margin.top + ')');
 
    // Scales
    
    /* Scales the months */
    var x0 = d3.scaleBand()
    	.domain(data.map( d => d.label))
    	.range([0, width])
    	.padding(0.2)
    
    /* Scales the current and prior periods in each month */
    var x1 = d3.scaleBand()
    	.domain(keys)
    	.range([0, x0.bandwidth()])
    	.padding(0.1)
    
    /* Scales the y axis based on request values */
    var y = d3.scaleLinear()
    		.domain([0, getRequestMax()])
        .range([height, 0])

function _buildChart() {
      var chart = g.append('g')
      	.classed('chart', true)
      
      // Adds a `g` element for every month,
      // which will contain 2 `rects` for data of each period
      var months = chart.selectAll('g')
      	.data(data)
      	.enter().append('g')
      		.attr('transform', function(d) {
        		return 'translate(' + x0(d.label) + ', 0)';
      		})
				.selectAll('rect')
      	.data(function(d) {
					var datum = keys.map(function(key, i) {
            return {
              key: key,
              fill: getFill(i),
              val: d.requests_submitted[key]
            }
          });
          return datum
        })
      	.enter().append('rect')
      	.attr('x', d => x1(d.key))
      	.attr('y', d => y(d.val))
      	.attr('width', x1.bandwidth())
      	.attr('height', d => height - y(d.val))
				.attr('fill', d => d.fill)
    }
    
    
 
    function _buildAxes() {
      var xAxis = d3.axisBottom(x0)
      var yAxis = d3.axisLeft(y)
      
      g.append('g')
      	.classed('axis', true)
      	.attr('transform', 'translate(0,' + height +')')
      	.call(xAxis)
      
      g.append('g')
      	.classed('axis', true)
      	.attr('transform', 'translate(0, 0)')
      	.call(yAxis)
    }
   
    function init() {
      
      _buildChart()
      _buildAxes()
      
    }
  
    init()
   
    // HELPERS
    
    
    /*
     * Takes an index of 0 or 1 returns a color depending on the index
     * @returns {string}  - A color string
     */
    
    function getFill(i) {
      return i ? 'lightblue' : 'lightgreen'
    } 
    
    
    /*
     * A wrapper around the d3.max method that returns the highest request submission
     * total for a given month from 2 separate years
     * ---
     * @returns {num}
     */
    
    function getRequestMax() {
    	return d3.max(getAllRequests(data))
    }
    
   
    /*
     * Combines all request submission values into an array
     * ---
     * @param   {Array} `data`  - An array of objects for each month
     * @returns {Array}
     */
    
    function getAllRequests(data) {
      return data.reduce(function(collection, d) {
        return collection.concat(
          keys.map(function(key) {
            return d.requests_submitted[key]                                
          })
        )
      }, [])
    }

</script>
</body>

https://d3js.org/d3.v4.min.js