These bars are nicely grouped but they should be stacked

The bars here are nicely grouped ordinally by "saif" (top level item) categories which is something I actually wanted to achieve but the bars in each of these ordinal categories overlap rather than stack. I made them 0.2 transparent so this can be easily tracked. I would love to get some guidance as to how may I be able to aggregate the total per 'saif' and to stack them accordingly.

<!DOCTYPE html>
<html lang="en">
	<head>
		<meta charset="utf-8">
		<title>Top items in the 2016 Israeli budget</title>
		<script type="text/javascript" src="https://d3js.org/d3.v3.js"></script>
		<style type="text/css">

@import url(https://fonts.googleapis.com/earlyaccess/alefhebrew.css);

body {
				background-color: white;
        font-family: "Alef Hebrew",
               "Helvetica Neue",
               Helvetica,
               Arial,
               sans-serif;
			}

h1 {
				font-size: 24px;
				margin: 0;
			}

p {
				font-size: 14px;
				margin: 10px 0 0 0;
			}

svg {
				background-color: white;
			}

rect {
				opacity: 0.2;
			}

rect:hover {
				fill: orange;
				opacity: 1;
			}

.axis path,
			.axis line {
				fill: none;
				stroke: black;
				shape-rendering: crispEdges;
			}

.axis text {
				font-size: 11px;
			}

.y.axis path,
			.y.axis line {
				opacity: 0;
			}

</style>
	</head>
	<body>

<h1>Top Level Items in the Israeli Budget</h1>

<p>These are the top level items in the Israeli budget in yet another senseless barchart on the way to becoming a slightly more useful infographic as the course progresses. Source: <a href="https://data.obudget.org/queries/225/source">oBudget.org</a>, 2015</p>

var w = 1000;
			var h = 1800;
			var padding = [ 20, 10, 30, 160 ];  //Top, right, bottom, left

var widthScale = d3.scale.linear()
								.range([ 0, w - padding[1] - padding[3] ]);

var heightScale = d3.scale.ordinal()
								.rangeRoundBands([ padding[0], h - padding[2] ], 0.1);

var xAxis = d3.svg.axis()
							.scale(widthScale)
							.orient("bottom");

var yAxis = d3.svg.axis()
							.scale(heightScale)
							.orient("left");

var svg = d3.select("body")
						.append("svg")
						.attr("width", w)
						.attr("height", h);

d3.csv("top_level_2016.csv", function(data) {

data.sort(function(a, b) {
					return d3.descending(+a.sum*1000, +b.sum*1000);  //original items are in thousands
				});

widthScale.domain([ 0, d3.max(data, function(d) {
					return +d.sum*1000;  //original items are in thousands
				}) ]);

heightScale.domain(data.map(function(d) { return d.saif; } ));

var rects = svg.selectAll("rect")
								.data(data)
								.enter()
								.append("rect");

rects.attr("x", padding[3])
					.attr("y", function(d) {
						return heightScale(d.saif);
					})
					.attr("width", function(d) {
						return widthScale(d.sum*1000);
					})
					.attr("height", heightScale.rangeBand())
					.attr("fill", "steelblue")
					.append("title")
					.text(function(d) {
						return "תקציב 2016 של \"" + d.saif + "\" (מסוג " + d.class_top + " / " + d.class_full  + ") הוא ₪" + d.sum * 1000;
					});

svg.append("g")
					.attr("class", "x axis")
					.attr("transform", "translate(" + padding[3] + "," + (h - padding[2]) + ")")
					.call(xAxis);

svg.append("g")
					.attr("class", "y axis")
					.attr("transform", "translate(" + padding[3] + ",0)")
					.call(yAxis);

});

</script>

</body>
</html>

Modified http://d3js.org/d3.v3.js to a secure url

https://d3js.org/d3.v3.js