meteor d3 import data - d3.js

Question

I'm having difficulties in importing data from MongoDB into the D3 heatmap function. An example can be found here: http://bl.ocks.org/tjdecke/5558084
The data looks like this:
[Object { _id="DoaB3wnW7CCzxqzug", weekday=1, hour=1, value=12}, Object { _id="RnwxHbRPGdWHBaAYu", weekday=1, hour=2, value=4}]
My code looks like this:
Template.weeklyUsageHeatmap.rendered = function() {
var margin = { top: 20, right: 0, bottom: 100, left: 20 },
width = 720 - margin.left - margin.right,
height = 340 - margin.top - margin.bottom,
gridSize = Math.floor(width / 24),
legendElementWidth = gridSize*2,
buckets = 9,
colors = ["#ffffe5","#fff7bc","#fee391","#fec44f","#fe9929","#ec7014","#cc4c02","#993404","#662506"]
days = ["Mo", "Tu", "We", "Th", "Fr", "Sa", "Su"],
times = ["1a", "2a", "3a", "4a", "5a", "6a", "7a", "8a", "9a", "10a", "11a", "12a", "1p", "2p", "3p", "4p", "5p", "6p", "7p", "8p", "9p", "10p", "11p", "12p"];
var data = WeeklyUsage.find().fetch();
d3.json(data,
function(d) {
return {
day: +d.weekday,
hour: +d.hour,
value: +d.value
};
},
function(error, data) {
var colorScale = d3.scale.quantile()
.domain([0, buckets - 1, d3.max(data, function (d) { return d.value; })])
.range(colors);
var svg = d3.select("#chart").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 dayLabels = svg.selectAll(".dayLabel")
.data(days)
.enter().append("text")
.text(function (d) { return d; })
.attr("x", 0)
.attr("y", function (d, i) { return i * gridSize; })
.style("text-anchor", "end")
.attr("transform", "translate(-6," + gridSize / 1.5 + ")")
.attr("class", function (d, i) { return ((i >= 0 && i <= 4) ? "dayLabel mono axis axis-workweek" : "dayLabel mono axis"); });
var timeLabels = svg.selectAll(".timeLabel")
.data(times)
.enter().append("text")
.text(function(d) { return d; })
.attr("x", function(d, i) { return i * gridSize; })
.attr("y", 0)
.style("text-anchor", "middle")
.attr("transform", "translate(" + gridSize / 2 + ", -6)")
.attr("class", function(d, i) { return ((i >= 7 && i <= 16) ? "timeLabel mono axis axis-worktime" : "timeLabel mono axis"); });
var heatMap = svg.selectAll(".hour")
.data(data)
.enter().append("rect")
.attr("x", function(d) { return (d.hour - 1) * gridSize; })
.attr("y", function(d) { return (d.weekday - 1) * gridSize; })
.attr("rx", 4)
.attr("ry", 4)
.attr("class", "hour bordered")
.attr("width", gridSize)
.attr("height", gridSize)
.style("fill", colors[0]);
heatMap.transition().duration(1000)
.style("fill", function(d) { return colorScale(d.value); });
heatMap.append("title").text(function(d) { return d.value; });
var legend = svg.selectAll(".legend")
.data([0].concat(colorScale.quantiles()), function(d) { return d; })
.enter().append("g")
.attr("class", "legend");
legend.append("rect")
.attr("x", function(d, i) { return legendElementWidth * i; })
.attr("y", height)
.attr("width", legendElementWidth)
.attr("height", gridSize / 2)
.style("fill", function(d, i) { return colors[i]; });
legend.append("text")
.attr("class", "mono")
.text(function(d) { return "≥ " + Math.round(d); })
.attr("x", function(d, i) { return legendElementWidth * i; })
.attr("y", height + gridSize);
});
};
The error I got was: d is NULL.