D3.js binding an object to data and appending for each key

Question

I'm a D3.js newbie and I'm learning how to play around with data.

Let's say I have an object with names as keys, and each key has an array of numbers like this:

var userdata = {
    'John' : [0, 1, 3, 9, 8, 7],
    'Harry': [0, 10, 7, 1, 1, 11],
    'Steve': [3, 1, 4, 4, 4, 17],
    'Adam' : [4, 77, 2, 13, 11, 13]
};

For each user, I would like to append an SVG object and then plot the line with the array of values for that user.

So here is my assumption of how that would look based on tutorials, but I know it is incorrect. This is to show my limited knowledge and give better understanding of what I'm doing:

First I should create the line

var line = d3.svg.line().interpolate('basis');

Then I want to bind the data to my body and append an svg element for each key:

d3.select('body')
    .selectAll('svg')
    .data(userdata)
    .enter()
    .append('svg')
    .append(line)
    .x(function(d, i) { return i; })
    .y(function(d) { return d[i]; });

So am I close??

Answer 1

Here's another example similar to mbeasley's: http://jsfiddle.net/2N2rt/15/ which adds axis, colors, and flips the chart right side up. First, I massaged your data just a little bit. Works the best if you have an array of values and I used keys to make getting to the name and values properties easier.

var data = [
    {name: 'John', values: [0,1,3,9, 8, 7]},
    {name: 'Harry', values: [0, 10, 7, 1, 1, 11]},
    {name: 'Steve', values: [3, 1, 4, 4, 4, 17]},
    {name: 'Adam', values: [4, 77, 2, 13, 11, 13]}
];

Generally in d3 you set up the chart like this which determines the size of the actual graph and the surrounding margins.

var margin = {top: 20, right: 80, bottom: 30, left: 50},
    width = 640 - margin.left - margin.right,
    height = 380 - margin.top - margin.bottom;

Then you can create your scales based on your data. Though you don't have to create them, they make positioning elements on the chart much easier by converting values to points. The domain is the min and max values that your data has, while the range is the min and max values of the size of your graph. Note that the range for the y axis gets reversed which puts (0,0) in the bottom left hand corner (usually 0 for the y axis is at the top).

var x = d3.scale.linear()
    .domain([0, d3.max(data, function(d) { return d.values.length - 1; })])
    .range([0, width]);

var y = d3.scale.linear()
    .domain([d3.min(data, function(d) { return d3.min(d.values); }),
             d3.max(data, function(d) { return d3.max(d.values); })])
    .range([height, 0]);

d3 has a couple of features for automatically creating color scales for you. Just set it up with the keys that you want to use and the number of colors (I think there are 10 and 20 color options).

var color = d3.scale.category10()
    .domain(d3.keys(data[0]).filter(function(key) { return key === "name"; }));

Here we use our scales to setup the x and y axis. There are lots of different options for the axes. TickFormat is handy to change how the ticks look, d3.format has lots of different options so that you rarely have to create a custom formatter.

var xAxis = d3.svg.axis()
    .scale(x)
    .tickFormat(d3.format('d'))
    .orient("bottom");

var yAxis = d3.svg.axis()
    .scale(y)
    .orient("left");

Here we create a reusable function that knows how to draw a line. We'll pass each datapoint for each person into the line function later on. Note that d holds the current set of values that we are graphing and i is the index of the current values within our original data array.

var line = d3.svg.line()
    .interpolate("basis")
    .x(function(d, i) { return x(i); })
    .y(function(d, i) { return y(d); });

Finally we can start adding things to our chart, first creating and positioning the main svg element.

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 + ")");

Then appending each axis.

  svg.append("g")
      .attr("class", "x axis")
      .attr("transform", "translate(0," + height + ")")
      .call(xAxis);

  svg.append("g")
      .attr("class", "y axis")
      .call(yAxis);

We append a group (g) for each person and associate the data with the group. We'll use this data when we draw the lines. We provide a class so that we can style the chart later if we want to.

  var people = svg.selectAll(".people")
      .data(data)
    .enter().append("g")
      .attr("class", "people");

Then finally draw the lines for each person.

  people.append("path")
      .attr("class", "line")
      .attr("d", function(d) { return line(d.values); })
      .style("stroke", function(d) { return color(d.name); });

I used your data to answer another question about drawing a grid when you have negative values. You can see that graph at http://jsfiddle.net/2y3er/2/.