Accessing nested JSON structure in D3 - json

Question

I'm trying to create a set of graphs using D3 and I'm having trouble figuring out how to access nested data structures in my JSON. The data looks something like this (truncated):
{ "date": "20120927",
"hours": [
{ "hour": 0, "hits": 823896 },
{ "hour": 1, "hits": 654335 },
{ "hour": 2, "hits": 548812 },
{ "hour": 3, "hits": 512863 },
{ "hour": 4, "hits": 500639 }
],
"totalHits": "32,870,234",
"maxHits": "2,119,767",
"maxHour": 12,
"minHits": "553,821",
"minHour": 3 }
{ "date": "20120928",
"hours": [
{ "hour": 0, "hits": 1235923 },
{ "hour": 1, "hits": 654335 },
{ "hour": 2, "hits": 1103849 },
{ "hour": 3, "hits": 512863 },
{ "hour": 4, "hits": 488506 }
],
"totalHits": "32,870,234",
"maxHits": "2,119,767",
"maxHour": 12,
"minHits": "553,821",
"minHour": 3 }
What I eventually want to do is create multiple radar graphs, one for each day, plotting the hits for each hour. But I'm having trouble even just getting inside the "hours" array. I can , for example, get a list of all the dates, like so:
d3.select("body")
.append("ul")
.selectAll("li")
.data(data)
.enter()
.append("li")
.text(function (d,i) {
return d.date;
});
But I can't get at anything more nested. Can someone help point me in the right direction?

Answer 1

Assuming data is an two-element array containing your two objects, you could do something like this:
d3.select("body").append("ul").selectAll("li")
.data(data) // top level data-join
.enter().append("li")
.each(function() {
var li = d3.select(this);
li.append("p")
.text(function(d) { return d.date; });
li.append("ul").selectAll("li")
.data(function(d) { return d.hours; }) // second level data-join
.enter().append("li")
.text(function(d) { return d.hour + ": " + d.hits; });
});
This would give you a nested structure like this:
<ul>
<li>
<p>20120927</p>
<ul>
<li>0: 823896</li>
<li>1: 654335</li>
<li>2: 548812</li>
<li>3: 512863</li>
<li>4: 500639</li>
</ul>
</li>
<li>
<p>20120928</p>
<ul>
<li>0: 1235923</li>
<li>1: 654335</li>
<li>2: 1103849</li>
<li>3: 512863</li>
<li>4: 488506</li>
</ul>
</li>
</ul>
See also Mike's Nested Selections article.