Question

I’m currently assembling some line graphs with circles at the datapoints from arrays of JSON objects formatted like so:

var data = [{
    "name": "metric1",
    "datapoints": [
        [10.0, 1333519140],
        [48.0, 1333519200]
    ]
}, {
    "name": "metric2",
    "datapoints": [
        [48.0, 1333519200],
        [12.0, 1333519260]
    ]
}]

I want to have a color for each metric, so I’m trying to color them based on the index of
the object within the array data. The code I have currently for just placing the circles looks like:

// We bind an svg group to each metric.
var metric_groups = this.vis.selectAll("g.metric_group")
  .data(data).enter()
  .append("g")
    .attr("class", "metric_group");

// Then bind a circle for each datapoint.
var circles = metric_groups.selectAll("circle")
.data(function(d) { return d.datapoints; });

circles.enter().append("circle")
  .attr("r", 3.5);

Now if I change that last bit to something like:

circles.enter().append("circle")
  .attr("r", 3.5);
  .style("fill", function(d,i) { return i%2 ? "red" : "blue"; }

I get alternating red and blue circles, as could be expected.
Taking some advice from Nested Selections : ‘Nesting and Index’, I tried:

circles.enter().append("circle")
  .attr("r", 3.5);
  .style("fill", function(d,i,j) { return j%2 ? "red" : "blue"; }

Which doesn’t work (j is undefined), presumably because we are in the named property datapoints, rather than an array element. How might I go about doing the coloring that I want without changing my data structure? Thanks!

Answer 1

The easiest thing to do here is to have the circles inherit a fill style from the parent G element:

var color = d3.scale.category20();

var metricGroup = vis.selectAll(".metric-group")
    .data(data)
  .enter().append("g")
    .attr("class", "metric-group")
    .style("fill", function(d) { return color(d.name); });

var circle = metricGroup.selectAll("circle")
    .data(function(d) { return d.datapoints; })
  .enter().append("circle")
    .attr("r", 3.5);

If you defined categorical colors as CSS classes, you could also use a dynamic class name and inherit that way:

var metricGroup = vis.selectAll(".metric-group")
    .data(data)
  .enter().append("g")
    .attr("class", function(d) { return "metric-group " + color(d.name); });

With corresponding CSS:

.metric1 circle { fill: red; }
.metric2 circle { fill: blue; }

Another approach is to use each to access the parent data:

metricGroup.each(function(p, j) {
  d3.select(this).selectAll("circle")
      .data(p.datapoints)
    .enter().append("circle")
      .attr("r", 3.5)
      .style("fill", color(p.name));
});

I also think using the group index j would work; I’m not sure why it’s undefined for you, but there is a spurious semicolon in your code example (in .attr("r", 3.5);), so it’s possible there’s something else going on. At any rate, it’s more idiomatic to derive categorical colors from data rather than group index, so I’d use one of the above techniques.