An issue i'm having with the d3 update pattern

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>US Map of Nielsen Media Markets</title>
  
</head>
<body>

<button data-month="Jan">
  January
</button>
<button data-month="Feb">
  February
</button>
<div id="toggle">
	<p>Initialized with "Jan", use buttons to toggle</p>
</div>
  
<script src="https://d3js.org/d3.v4.min.js"></script>
<script>
//define some data
var data = [
		{"location": 1, "month": "Jan", "year": 2017, "value": "#ccc"},
		{"location": 2, "month": "Jan", "year": 2017, "value": "#999"},
		{"location": 3, "month": "Jan", "year": 2017, "value": "#666"},
		{"location": 4, "month": "Jan", "year": 2017, "value": "#333"},
		{"location": 1, "month": "Jan", "year": 2018, "value": "#fcc"},
		{"location": 2, "month": "Jan", "year": 2018, "value": "#f99"},
		{"location": 3, "month": "Jan", "year": 2018, "value": "#f66"},
    {"location": 4, "month": "Jan", "year": 2018, "value": "#f33"},
		{"location": 1, "month": "Feb", "year": 2017, "value": "#cfc"},
		{"location": 2, "month": "Feb", "year": 2017, "value": "#9f9"},
		{"location": 3, "month": "Feb", "year": 2017, "value": "#6f6"},
		{"location": 4, "month": "Feb", "year": 2017, "value": "#3f3"},
		{"location": 1, "month": "Feb", "year": 2018, "value": "#ccf"},
		{"location": 2, "month": "Feb", "year": 2018, "value": "#99f"},
		{"location": 3, "month": "Feb", "year": 2018, "value": "#66f"},
		{"location": 4, "month": "Feb", "year": 2018, "value": "#33f"},
	]
 
 // nest the data by month then year
 var by_month = d3.nest()
		.key(function(d){return d.month})
		.key(function(d){return d.year})
		.entries(data)
    
    
function render(ident, key_month) {

var w = 300
	var h = 100

var container = d3.select(ident)
	
  // select and add a container div for each year in the data
  // using only the data for the target month
	var year = container
		.selectAll("div.year")
		
	var yearEnter = year.data(by_month.filter(function(d){return d.key == key_month})[0].values)
		.enter()
		.append("div")
		.attr("class", "year")

// select and add a h2
	var h2 = year.select("h2")
	var h2Enter = yearEnter.append('h2')
  
	//update the text	
	h2Enter.merge(h2)
		.text(function(d){return d.values[0].month + " " + d.values[0].year})
	
  // select and add a svg
	var svg = year.select("svg")
	var svgEnter = yearEnter.append("svg")
	
  //update the svg dimensions
	svgEnter.merge(svg)
		.attr("width", w)
		.attr("height", h)
	
  // select and add an outer wrapper g	
	var locations_wrap = svg.select("g.locations")
	var locations_wrapEnter = svgEnter.append("g")
		.attr('class', "locations")
	
  // select and add element wrapper g
	var location = locations_wrap.selectAll("g.location")
	var locationEnter = locations_wrapEnter.selectAll("g.location")
		.data(function(d){return d.values})
		.enter()
		.append("g")
		.attr("class", "site")
    
  // merge and position element wrappers
	var locationMerged = locationEnter.merge(location)
		.attr("transform", function(d,i){return "translate("+[w/4*i]+")"})
	
  //select and add rects inside element
	var rects = location.select("rect")
	var rectsEnter = locationEnter.append("rect")
		.attr("x", 5)
		.attr("width", w/4 - 5)
		.attr("height", w/4 - 5)
	
  // ???? merge the rects and update its fill ????
  rectsEnter.merge(rects).attr("fill", function(d){return d.value})
}

// on button click change the subset of data being used
d3.selectAll("button").on("click", function(){
    render("#toggle",this.getAttribute("data-month"))
})

render("#toggle","Jan")
  
</script>

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