Why is domain not using d3.max(data) in D3?

Question

I'm new to D3 and playing around with a scatterplot. I cannot get d3.max(data) to work correctly in setting up domain!

I have the following setting up a random dataset:

var data = [];
      for (i=0; i < 40; i++){
            data.push({"x": i/40, "y": i/8, "a": Math.floor(Math.random() * 3), "x2": Math.random()});
        }

And then the following to set my coordinates:

 var x = d3.scale.linear().domain([0, 1]).range([0 + margin, w-margin]),
            y = d3.scale.linear().domain([0, d3.max(data)]).range([0 + margin, h-margin]),
            c = d3.scale.linear().domain([0, 3]).range(["hsl(100,50%,50%)", "rgb(350, 50%, 50%)"]).interpolate(d3.interpolateHsl);

This puts all 40 points in a single, horizontal line. If I replace d3.max(data) with '5' then it is a diagonal (albeit from the upper left to the bottom right, I'm still struggling to flip y-coordinates). Why isn't d3.max(data) working as expected?

Answer 1

d3.max() expects an array of numbers, not of objects. The elements of data have an internal key-value structure and there is no way for d3.max() to know what to take the maximum of. You can use something like jQuery's $.map to get the elements of the objects you want and then take the max, e.g.

var maxy = d3.max($.map(data, function(d) { return d.y; }));

Edit:

As pointed out in the comment below, you don't even need JQuery for this, as .map() is a native Array method. The code then becomes simply

var maxy = d3.max(data.map(function(d) { return d.y; }));

or even simpler (and for those browsers that don't implement Array.map()), using the optional second argument of d3.max that tells it how to access values within the array

var maxy = d3.max(data, function(d) { return d.y; });