How to transition area along one axis in D3.js - d3.js

Question

I am trying do some path interpolation in D3. I'd like to produce an area plot like this, but I want to transition the area along the y-axis, starting from the bottom of the xaxis up to the final position shown in the example. Here's a quick sketch to explain what I'd like to do:
I'd like to start the transition with no area:
and transition it up along the y-axis:
Using the code, copied from the example, here's what I'd trying to do:
var margin = {top: 20, right: 20, bottom: 30, left: 50},
width = 960 - margin.left - margin.right,
height = 500 - margin.top - margin.bottom;
var parseDate = d3.time.format("%d-%b-%y").parse;
var x = d3.time.scale()
.range([0, width]);
var y = d3.scale.linear()
.range([height, 0]);
var xAxis = d3.svg.axis()
.scale(x)
.orient("bottom");
var yAxis = d3.svg.axis()
.scale(y)
.orient("left");
var area = d3.svg.area()
.x(function(d) { return x(d.date); })
.y0(height)
.y1(function(d) { return y(d.close); });
var svg = d3.select("body").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 + ")");
d3.tsv("data.tsv", function(error, data) {
data.forEach(function(d) {
d.date = parseDate(d.date);
d.close = +d.close;
});
x.domain(d3.extent(data, function(d) { return d.date; }));
y.domain([0, d3.max(data, function(d) { return d.close; })]);
var dataSel = svg.selectAll('.area').data(data)
dataSel.exit().remove()
dataSel.enter()
.append('path')
.attr("class", "area")
.attr("d", 'M0,0h' + width) // my idea here was to draw a path that
// has no area along the x-axis and then
// interpolate the path up to the final area
dataSel.transition() // transition the path to its final position
.duration(1000)
.attr("d", area)
svg.append("g")
.attr("class", "x axis")
.attr("transform", "translate(0," + height + ")")
.call(xAxis);
svg.append("g")
.attr("class", "y axis")
.call(yAxis)
.append("text")
.attr("transform", "rotate(-90)")
.attr("y", 6)
.attr("dy", ".71em")
.style("text-anchor", "end")
.text("Price ($)");
});
Can anyone explain/show how I can transition a path along one axis as I'm trying to do?

Answer 1

D3 path transitions only really work if the starting and ending paths have the same number of control points. So, for example, D3 can't transition a line into an area. What you could do, however, is something like the following:
Use d3.area to generate the final path for the area.
Make a copy of the path and, in the copy, change all the control points that correspond to the "top" of the area to control points on the "bottom" of the area. (In other words, modify their y-values.)
Draw the area using this modified path.
Transition to the final path.