Set arbitrary center in streamgraph offset - d3.js

Question

I have data in a streamgraph stack layout and the desired aesthetic I'm after is to assign an arbitrary series as the center line(s). Series above those lines (as determined by their index in the data) will stack on top and series below that line will stack below.
Here's a jsFidde.
In this example, I'd like the MS and the RC series to be single horizontal lines with the other groups stacked above and below them, respectively. (As opposed to the data index, I could also set the middle series based on some data attribute, in this example, oldest date would make sense.)
I think the solution would require passing my own offset function but I'm having a hard time figuring out how the built-in ones do what they do.
HTML
<div class="chart">
JS
// Adapted from https://gist.github.com/WillTurman/4631136
var data = [
{"key":"DJ","value":0,"date":"1/8/13"},
{"key":"DJ","value":0,"date":"1/9/13"},
{"key":"DJ","value":0,"date":"1/10/13"},
{"key":"DJ","value":1,"date":"1/11/13"},
{"key":"DJ","value":1,"date":"1/12/13"},
{"key":"MS","value":0,"date":"1/8/13"},
{"key":"MS","value":1,"date":"1/9/13"},
{"key":"MS","value":1,"date":"1/10/13"},
{"key":"MS","value":1,"date":"1/11/13"},
{"key":"MS","value":1,"date":"1/12/13"},
{"key":"RC","value":0,"date":"1/8/13"},
{"key":"RC","value":1,"date":"1/9/13"},
{"key":"RC","value":1,"date":"1/10/13"},
{"key":"RC","value":1,"date":"1/11/13"},
{"key":"RC","value":1,"date":"1/12/13"},
{"key":"CG","value":0,"date":"1/8/13"},
{"key":"CG","value":0,"date":"1/9/13"},
{"key":"CG","value":0,"date":"1/10/13"},
{"key":"CG","value":0,"date":"1/11/13"},
{"key":"CG","value":1,"date":"1/12/13"},
{"key":"RI","value":0,"date":"1/8/13"},
{"key":"RI","value":0,"date":"1/9/13"},
{"key":"RI","value":0,"date":"1/10/13"},
{"key":"RI","value":0,"date":"1/11/13"},
{"key":"RI","value":1,"date":"1/12/13"}
]
chart(data, "pink");
var datearray = [];
var colorrange = [];
function chart(data, color) {
if (color == "blue") {
colorrange = ["#045A8D", "#2B8CBE", "#74A9CF", "#A6BDDB", "#D0D1E6", "#F1EEF6"];
}
else if (color == "pink") {
colorrange = ["#980043", "#DD1C77", "#DF65B0", "#C994C7", "#D4B9DA", "#F1EEF6"];
}
else if (color == "orange") {
colorrange = ["#B30000", "#E34A33", "#FC8D59", "#FDBB84", "#FDD49E", "#FEF0D9"];
}
strokecolor = colorrange[0];
var format = d3.time.format("%m/%d/%y");
var margin = {top: 20, right: 40, bottom: 30, left: 30};
var width = document.body.clientWidth - margin.left - margin.right;
var height = 400 - margin.top - margin.bottom;
var tooltip = d3.select("body")
.append("div")
.attr("class", "remove")
.style("position", "absolute")
.style("z-index", "20")
.style("visibility", "hidden")
.style("top", "30px")
.style("left", "55px");
var x = d3.time.scale()
.range([0, width]);
var y = d3.scale.linear()
.range([height-10, 0]);
var z = d3.scale.ordinal()
.range(colorrange);
var xAxis = d3.svg.axis()
.scale(x)
.orient("bottom")
.ticks(d3.time.weeks);
var yAxis = d3.svg.axis()
.scale(y);
var yAxisr = d3.svg.axis()
.scale(y);
var stack = d3.layout.stack()
.offset("silhouette")
.values(function(d) { return d.values; })
.x(function(d) { return d.date; })
.y(function(d) { return d.value; });
var nest = d3.nest()
.key(function(d) { return d.key; });
var area = d3.svg.area()
.interpolate("linear")
.x(function(d) { return x(d.date); })
.y0(function(d) { return y(d.y0); })
.y1(function(d) { return y(d.y0 + d.y); });
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 + ")");
data.forEach(function(d) {
d.date = format.parse(d.date);
d.value = +d.value;
});
var layers = stack(nest.entries(data));
x.domain(d3.extent(data, function(d) { return d.date; }));
y.domain([0, d3.max(data, function(d) { return d.y0 + d.y; })]);
svg.selectAll(".layer")
.data(layers)
.enter().append("path")
.attr("class", "layer")
.attr("d", function(d) { return area(d.values); })
.style("fill", function(d, i) { return z(i); });
svg.append("g")
.attr("class", "x axis")
.attr("transform", "translate(0," + height + ")")
.call(xAxis);
svg.append("g")
.attr("class", "y axis")
.attr("transform", "translate(" + width + ", 0)")
.call(yAxis.orient("right"));
svg.append("g")
.attr("class", "y axis")
.call(yAxis.orient("left"));
svg.selectAll(".layer")
.attr("opacity", 1)
.on("mouseover", function(d, i) {
svg.selectAll(".layer").transition()
.duration(250)
.attr("opacity", function(d, j) {
return j != i ? 0.6 : 1;
})})
.on("mousemove", function(d, i) {
mousex = d3.mouse(this);
mousex = mousex[0];
var invertedx = x.invert(mousex);
invertedx = invertedx.getMonth() + invertedx.getDate();
var selected = (d.values);
for (var k = 0; k < selected.length; k++) {
datearray[k] = selected[k].date
datearray[k] = datearray[k].getMonth() + datearray[k].getDate();
}
mousedate = datearray.indexOf(invertedx);
pro = d.values[mousedate].value;
d3.select(this)
.classed("hover", true)
.attr("stroke", strokecolor)
.attr("stroke-width", "0.5px"),
tooltip.html( "<p>" + d.key + "<br>" + pro + "</p>" ).style("visibility", "visible");
})
.on("mouseout", function(d, i) {
svg.selectAll(".layer")
.transition()
.duration(250)
.attr("opacity", "1");
d3.select(this)
.classed("hover", false)
.attr("stroke-width", "0px"), tooltip.html( "<p>" + d.key + "<br>" + pro + "</p>" ).style("visibility", "hidden");
})
var vertical = d3.select(".chart")
.append("div")
.attr("class", "remove")
.style("position", "absolute")
.style("z-index", "19")
.style("width", "1px")
.style("height", "380px")
.style("top", "10px")
.style("bottom", "30px")
.style("left", "0px")
.style("background", "#fff");
d3.select(".chart")
.on("mousemove", function(){
mousex = d3.mouse(this);
mousex = mousex[0] + 5;
vertical.style("left", mousex + "px" )})
.on("mouseover", function(){
mousex = d3.mouse(this);
mousex = mousex[0] + 5;
vertical.style("left", mousex + "px")});
}