D3 Add Dots to Stacked Area Chart - d3.js

Question

I need to add dots to the stacked area chart in the code below. I've tried numerous iterations in the code with .data(layers), but it errors out when looking for cx and cy on all the iterations I've tried. The main issue is that i don't understand how to drill into the layers variable to get the cummulative summation so the circles match the lines in the stacked area.
Here's a fiddle, and heres a snippet:
var data = d3.csv.parse(d3.select("#dataset").text());
d3.select("#dataset").remove();
var format = d3.time.format("%m/%d/%y");
var margin = {top: 20, right: 30, bottom: 30, left: 40},
width = 500 - margin.left - margin.right,
height = 500 - margin.top - margin.bottom;
var x = d3.time.scale()
.range([0, width]);
var y = d3.scale.linear()
.range([height, 0]);
var z = d3.scale.category20c();
var xAxis = d3.svg.axis()
.scale(x)
.orient("bottom")
.ticks(d3.time.days);
var yAxis = d3.svg.axis()
.scale(y)
.orient("left");
var stack = d3.layout.stack()
.offset("zero")
.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("cardinal")
.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("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 + ")");
var mygroups = d3.map(data, function(d){return(d.key)}).keys()
var color = d3.scale.ordinal()
.domain(mygroups)
.range(['#CA999A','#99A3B0','#9FBD9F'])
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 color(i); })
.style("stroke","black");
svg.append("g")
.attr("class", "x axis")
.attr("transform", "translate(0," + height + ")")
.call(xAxis)
.selectAll("path","line")
.style({
fill: "none",
stroke: "#000",
"shape-rendering":"crispEdges"
});
svg.append("g")
.attr("class", "y axis")
.call(yAxis)
.selectAll("path","line")
.style({
fill: "none",
stroke: "#000",
"shape-rendering":"crispEdges"
});
//adds dots where original data would go but without error
/* svg.selectAll("circle")
.data(data)
.enter()
.append("circle")
.attr("class", "dot")
.attr("r", 5)
.attr("cx", function(d) { return x(d.date); })
.attr("cy", function(d) { return y(d.value); });
*/
//errors out with <circle> attribute cx: Expected length, "NaN".
//Tired various looping functions that would not work.
//I want the dots to follow the lines in the stack.
// it does at least put a dot on the graph
svg.selectAll("circle")
.data(layers)
.enter()
.append("circle")
.attr("class", "dot")
.attr("r", 5)
.attr("cx", function(d) { return x(d.date); })
.attr("cy", function(d) { return y(d.value); });
<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.17/d3.min.js"></script>
<pre id = "dataset">key,value,date
Group1,37,04/23/12
Group2,12,04/23/12
Group3,46,04/23/12
Group1,32,04/24/12
Group2,19,04/24/12
Group3,42,04/24/12
Group1,45,04/25/12
Group2,16,04/25/12
Group3,44,04/25/12
Group1,24,04/26/12
Group2,52,04/26/12
Group3,64,04/26/12</pre>

Answer 1

⚠️This question and answer both use d3v3 - d3v4+ stacks create a data array with a different structure - as such this answer may not be useful for d3v4+
Plotting circles with the data variable won't work, even if scaling, as the values returned will be for a non-cumulative line/area graph. Alternatively, plotting values in the layers variable won't work as it contains only one value per layer. Each layer in layers contains an array with all the points that belong to that layer, we can grab all of those points and plot those as circles.
Here's the layers structure:
[
{
"key": "Group1",
"values": [
{
"key": "Group1",
"value": 37,
"date": "2012-04-23T07:00:00.000Z",
"y0": 0,
"y": 37
},
...
{
"key": "Group1",
"value": 24,
"date": "2012-04-26T07:00:00.000Z",
"y0": 0,
"y": 24
}
]
},
{
"key": "Group2",
"values": [
{
"key": "Group2",
"value": 12,
"date": "2012-04-23T07:00:00.000Z",
"y0": 37,
"y": 12
},
... // an so on.
So, we can cycle through each item of layers, which means we cycle through each layer, and collect all the points. Something like this will suffice:
// Grab all coordinates of all layers:
var points = [];
layers.forEach(function(d) {
return points.push(...d.values);
})
Now, let's look at each of the items in our points array:
{
"key": "Group1",
"value": 37,
"date": "2012-04-23T07:00:00.000Z",
"y0": 0,
"y": 37
}
Here, y represents an items height, and y0 represents it's base (the bottom of the layer at that point). Since we only want to plot each point once, and we don't want to plot the 0 values of the bottom layer's y0 property, we should only plot the topmost y value (y+y0):
//adds dots where original data would go but without error
svg.selectAll("circle")
.data(points)
.enter()
.append("circle")
.attr("class", "dot")
.attr("r", 5)
.attr("cx", function(d) { return x(d.date); })
.attr("cy", function(d) { return y(d.y0+d.y); });
Which looks like this altogether:
var data = d3.csv.parse(d3.select("#dataset").text());
d3.select("#dataset").remove();
var format = d3.time.format("%m/%d/%y");
var margin = {top: 20, right: 30, bottom: 30, left: 40},
width = 500 - margin.left - margin.right,
height = 500 - margin.top - margin.bottom;
var x = d3.time.scale()
.range([0, width]);
var y = d3.scale.linear()
.range([height, 0]);
var z = d3.scale.category20c();
var xAxis = d3.svg.axis()
.scale(x)
.orient("bottom")
.ticks(d3.time.days);
var yAxis = d3.svg.axis()
.scale(y)
.orient("left");
var stack = d3.layout.stack()
.offset("zero")
.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("cardinal")
.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("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 + ")");
var mygroups = d3.map(data, function(d){return(d.key)}).keys()
var color = d3.scale.ordinal()
.domain(mygroups)
.range(['#CA999A','#99A3B0','#9FBD9F'])
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 color(i); })
.style("stroke","black");
svg.append("g")
.attr("class", "x axis")
.attr("transform", "translate(0," + height + ")")
.call(xAxis)
.selectAll("path","line")
.style({
fill: "none",
stroke: "#000",
"shape-rendering":"crispEdges"
});
svg.append("g")
.attr("class", "y axis")
.call(yAxis)
.selectAll("path","line")
.style({
fill: "none",
stroke: "#000",
"shape-rendering":"crispEdges"
});
// Grab all coordinates of all layers:
var points = [];
layers.forEach(function(d) {
return points.push(...d.values);
})
//adds dots where original data would go but without error
svg.selectAll("circle")
.data(points)
.enter()
.append("circle")
.attr("class", "dot")
.attr("r", 5)
.attr("cx", function(d) { return x(d.date); })
.attr("cy", function(d) { return y(d.y0+d.y); });
<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.17/d3.min.js"></script>
<pre id = "dataset">key,value,date
Group1,37,04/23/12
Group2,12,04/23/12
Group3,46,04/23/12
Group1,32,04/24/12
Group2,19,04/24/12
Group3,42,04/24/12
Group1,45,04/25/12
Group2,16,04/25/12
Group3,44,04/25/12
Group1,24,04/26/12
Group2,52,04/26/12
Group3,64,04/26/12</pre>