enter append flow class ex

<!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; }
  </style>
</head>

</svg> 
  <script>
var data = [39,112,186,286,372,286,192,108,54,40];
var rectWidth = 50
var height = 500
var svg = d3.select('svg')
var dmax = d3.max(data)
var ext =  d3.extent(data)
var rscale = d3.scaleLinear()
							.domain(ext)
							.range(0,50)

console.log(ext);
svg.selectAll('rect')
	.data(data)
	.enter().append('rect')
	.attr('x', function (d,i){return i*rectWidth})
	.attr('y', function (d,i){return height - d})
	.attr('width', rectWidth)
	.attr('height', function (d){return d})
	.attr('fill', 'steelblue')
	.attr('stroke','black')
;

svg.selectAll('circle')
	.data(data)
	.enter().append('circle')
	.attr('cx', function(d,i){return i*rectWidth + (rectWidth/2)})
	.attr('cy', function(d,i){return height - (d+(rectWidth/2)+20)})
	.attr("r", function(d,i) {console.log(rscale(d));return rscale(d) })
	.attr("fill", 'yellow')
	.attr('stroke','black')

</script>
</body>

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