How find the maximum of all arrays in an array of arrays? - d3.js

Question

I'm trying to learn D3 by book and examples. One example I'm working through is a simple (multi) line chart located here http://bl.ocks.org/mbostock/3884955#index.html .
I can follow along for the most part but I can't make sense of this:
y.domain([
d3.min(cities, function(c) { return d3.min(c.values, function(v) { return v.temperature; }); }),
d3.max(cities, function(c) { return d3.max(c.values, function(v) { return v.temperature; }); })
]);
When I was trying to write the code on my own, using the example as a cheat sheet, I came up with this
y.domain([0, d3.max(data, function(d) { return d.temperature; })]);
because I wanted the y range to span from 0 to the max of all temperatures.
I believe I have two questions here:
1) is the nested mins and maxs because it's looking at the max of each array within the array?
2) am I correct thinking that 'cities' is the entire array and values is the array of temperatures within 'cities'?
Apologies if this question isn't very focused. I believe I want to figure out how to find the maximum of an array of arrays.

Answer 1

Is the nested mins and maxs because it's looking at the max of each array within the array?
Yes! you are right the cities json is an array which has another array in it with key values the idea here is t find the min temperature in this nested array
d3.min(cities, function(c) { return d3.min(c.values, function(v) { return v.temperature; }); }),
am I correct thinking that 'cities' is the entire array and values is the array of temperatures within 'cities'?
Yes you are correct again copy this json below in a json formatter you will be able to understand the JSON better:
cities = [
{
"name":"New York",
"values":[
{
"date":"2011-09-30T18:30:00.000Z",
"temperature":63.4
},
{
"date":"2011-10-01T18:30:00.000Z",
"temperature":58
},
{
"date":"2011-10-02T18:30:00.000Z",
"temperature":53.3
},
{
"date":"2011-10-03T18:30:00.000Z",
"temperature":55.7
},
{
"date":"2011-10-04T18:30:00.000Z",
"temperature":64.2
}
]
},
{
"name":"San Francisco",
"values":[
{
"date":"2011-09-30T18:30:00.000Z",
"temperature":62.7
},
{
"date":"2011-10-01T18:30:00.000Z",
"temperature":59.9
},
{
"date":"2011-10-02T18:30:00.000Z",
"temperature":59.1
},
{
"date":"2011-10-03T18:30:00.000Z",
"temperature":58.8
}
]
},
{
"name":"Austin",
"values":[
{
"date":"2011-09-30T18:30:00.000Z",
"temperature":72.2
},
{
"date":"2011-10-01T18:30:00.000Z",
"temperature":67.7
},
{
"date":"2011-10-02T18:30:00.000Z",
"temperature":69.4
}
]
}
]
Hope this helps!