transition when adding new data to d3 streamgraph

I use d3 to draw a stream graph very similar to the official example http://bl.ocks.org/mbostock/4060954:

The only difference is how I updated it with new data. I don't only want a vertical (y-value) transition, but also want to add new data points on the right. The whole graph should become compressed in the horizontal direction.

It was no problem to achieve the desired result, the only problem is that the transition between the two states does not look as expected.

You can find a a minimal example of the strange transition effect on JSfiddle: http://jsfiddle.net/jaYJ9/4/

Press the update button to see the effect

test_data0 = [{"0": 0.0, "1": 0.0, "-1": 0.0}, {"0": 0.0, "1": 0.6, "-1": 0.0}, {"0": 0.0, "1": 0.3, "-1": 0.0}, {"0": 0.0, "1": 0.0, "-1": 0.6}, {"0": 0.3, "1": 0.0, "-1": 0.0}, {"0": 0.0, "1": 0.3, "-1": 0.3}, {"0": 0.3, "1": 0.0, "-1": 0.0}, {"0": 0.3, "1": 0.0, "-1": 0.0}, {"0": 0.0, "1": 0.0, "-1": 0.0}]
test_data1 = [{"0": 0.0, "1": 0.0, "-1": 0.0}, {"0": 0.0, "1": 0.6, "-1": 0.0}, {"0": 0.0, "1": 0.3, "-1": 0.0}, {"0": 0.0, "1": 0.0, "-1": 0.6}, {"0": 0.3, "1": 0.0, "-1": 0.0}, {"0": 0.0, "1": 0.3, "-1": 0.3}, {"0": 0.3, "1": 0.0, "-1": 0.0}, {"0": 0.3, "1": 0.0, "-1": 0.0}, {"0": 0.0, "1": 0.0, "-1": 0.0}, {"0": 0.0, "1": 0.0, "-1": 0.0}]

$('#update').click(function(){
    streamed_history(test_data1)
});
var width = 300,
    height = 200,
    colors = {'0': '#6ff500', '1': '#ffad0a', '-1': '#f90035'},
    feedbacks = [-1, 0, 1],
    stack = d3.layout.stack();
var svg = d3.select("#timeline").append("svg")
    .attr("width", width)
    .attr("height", height);
var y = d3.scale.linear()
    .domain([0, 1])
    .range([height, 0]);

streamed_history(test_data0)

function streamed_history(data) {
    data_array = feedbacks.map(function (f) {
        return data.map(function(element, i) { return {x: i, y: element[f]}; })
    }),
    layers = stack(data_array)
    layers = feedbacks.map(function (f, i) {
        return {layer: layers[i], feedback: f, color: colors[f]}
    })

    var x = d3.scale.linear()
        .domain([0, data.length - 1])
        .range([0, width]);

    var area = d3.svg.area().interpolate("basis")
        .x(function(d) { return x(d.x); })
        .y0(function(d) { return y(d.y0); })
        .y1(function(d) { return y(d.y0 + d.y); });

    //enter
    svg.selectAll("path")
        .data(layers)
      .enter().append("path")
        .attr("d", function (d) {return area(d.layer);})
        .style("fill", function(d) { return d.color; });

    //update
    d3.selectAll("path")
      .data(layers)
    .transition()
      .duration(2000)
      .attr("d", function (d) {return area(d.layer);});
}

This problem revolves around the fact that, for SVG animations, you can only push points onto the end of a path.

First, the fix (this will only work if the graphics are always vertically dense and have consistent ordering for which graph is highest):

...
var area = d3.svg.area().interpolate("basis")
    ...
    .y0(function(d) { return y(null); }) // The null here is super important!
    ...
...
// Add this function
function fixPath (path) {
    var Lidx = path.indexOf('L');
    var Cidx =  path.slice(Lidx).indexOf('C');
    var PCidx = path.slice(0,Lidx).lastIndexOf('C');

    var lp = path.substr(PCidx, Lidx-PCidx);
    var ss = path.substr(Lidx, Cidx);

    return (path.slice(0,Lidx) + lp + ss + path.slice(Lidx));
}
...
svg.selectAll("path")
    .data(layers.reverse()) // Gotta reverse the order!
    .attr("d", function (d) { return fixPath(area(d.layer)); }) // Have to double up the bottom right corner to avoid artifacts
    ...
...
d3.selectAll("path")
    .data(layers)
    .attr("d", function (d) { return fixPath(area(d.layer)); }) // When updating too!
    ...

Working example here: http://jsfiddle.net/f5JSR/2/

Now, the explanation...

Each of the color bands in your graph is a closed path, and d3.js constructs them so that none of these color bands overlaps with each other. The problem is, this means each of these paths starts at the bottom left corner, and loops around all the way back to itself. When you add a new point on these paths, you are adding it at the end, and it is pushing the rest of the path counter-clockwise around (creating that weird animation effect).

I initially tried solving this using SVG clipping and the fill-rule: evenodd property, but it seems that to use clipping you have to create compound paths, and adding new points pushes them on the end of this compound path (you can't, for example, push a point onto the first path of the compound path), so the problem persists.

Instead, this solution does away with d3.js's cleverness and instead makes all of your color bands expand to the bottom of the graph (that's what the y(null); line is doing). It then orders the paths so the highest ones are drawn first. This approach breaks down if one graph's height falls below another graph's height.

Finally, to when points are pushed around the bottom right corner, there can be some weird artifacting. To fix that, I double up the number of points at the bottom right corner using the fixPath function.

Anyways, this solution works for the example case you had. I hope this helps.