Combining Parent and Nested Data with d3.js

Question

I have a data structure like this (assume that the data structure is non-negotiable):

data = {
    segments : [
        {x : 20, size : 10,          


        

Answer 1

I would try to flatten the colors before you actually start creating the elements. If changes to the data occur I would then update this flattened data structure and redraw. The flattened data needs to be stored somewhere to make real d3 transitions possible.

Here is a longer example that worked for me. Yon can see it in action here.

Here is the code:

var data = {
    segments : [
        {x : 20, size : 10, colors : ['#ff0000','#00ff00']},
        {x : 40, size : 20, colors : ['#0000ff','#000000']}
    ]
};

function pos(d,i) { return d.x + (i * d.size); } // rect position
function size(d,i) { return d.size; }            // rect size
function f(d,i) { return d.color; }              // rect color

function flatten(data) {
    // converts the .colors to a ._colors list
    data.segments.forEach( function(s,i) {
        var list = s._colors = s._colors || [];
        s.colors.forEach( function(c,j) {
            var obj = list[j] = list[j] || {}
            obj.color = c
            obj.x = s.x
            obj.size = s.size
        });
    });
}

function changeRect(chain) {
    return chain
    .transition()
    .attr('x',pos)
    .attr('y',pos)
    .attr('width',size)
    .attr('height',size)
    .attr('fill',f)
    .style('fill-opacity', 0.5)
}

vis = d3
.select('#container')
.append('svg')
.attr('width',200)
.attr('height',200);

// add the top-level svg element and size it
function update(){

    flatten(data);

    // add the nested svg elements
    var all = vis.selectAll('g')
    .data(data.segments)

    all.enter().append('g');
    all.exit().remove();

    // Add a rectangle for each color
    var rect = all.selectAll('rect')
    .data(function (d) { return d._colors; }, function(d){return d.color;})

    changeRect( rect.enter().append('rect') )
    changeRect( rect )

    rect.exit().remove()
}

function changeLater(time) {
    setTimeout(function(){
        var ds = data.segments
        ds[0].x    = 10 + Math.random() * 100;
        ds[0].size = 10 + Math.random() * 100;
        ds[1].x    = 10 + Math.random() * 100;
        ds[1].size = 10 + Math.random() * 100;
        if(time == 500)  ds[0].colors.push("orange")
        if(time == 1000) ds[1].colors.push("purple")
        if(time == 1500) ds[1].colors.push("yellow")
        update()
    }, time)
}

update()
changeLater(500)
changeLater(1000)
changeLater(1500)

Important here is the flatten function which does the data conversion and stores/reuses the result as _colors property in the parent data element. Another important line is;

.data(function (d) { return d._colors; }, function(d){return d.color;})

which specifies where to get the data (first parameter) AND what the unique id for each data element is (second parameter). This helps identifying existing colors for transitions, etc.