How to fix shifted columns on histogram? - d3.js

Question

I am building a histogram with number of days as the x axis, but the columns are shifted. I want to bin the data on months, or more exactly, 30-day intervals. For some reason, when generating this graph the columns are not the right size, and the bars end up shifting a lot, as shown in the picture:
The bins are hardcoded like this:
var data = d3.layout.histogram()
.bins(20)
(values);
var y = d3.scale.linear()
.domain([0, d3.max(data, function(d) { return d.y; })])
.range([height, 0]);
var xAxis = d3.svg.axis()
.scale(x)
.tickValues([0,30,60,90,120,150,180,210,240,270,300,330,360,390,420,450,480,510,540,570,600])
.orient("bottom");
var svg = d3.select("#graph").append("svg")
.attr("width", width + margin.left + margin.right)
.attr("height", height + margin.top + margin.bottom)
.append("g")
.attr("transform", "translate(" + margin.left + "," + margin.top + ")");
var bar = svg.selectAll(".bar")
.data(data)
.enter().append("g")
.attr("class", "bar")
.attr("transform", function(d) { return "translate(" + x(d.x) + "," + y(d.y) + ")"; });
bar.append("rect")
.attr("x", 1)
.attr("width", x(data[0].dx) - 1)
.attr("height", function(d) { return height - y(d.y); });
bar.append("text")
.attr("dy", ".75em")
.attr("y", 6)
.attr("x", x(data[0].dx) / 2)
.attr("text-anchor", "middle")
.text(function(d) { return formatCount(d.y); });
svg.append("g")
.attr("class", "x axis")
.attr("transform", "translate(0," + height + ")")
.call(xAxis);
What is the correct way to solve this issue?