Alex Spangher Alex Spangher - 2 months ago 25
Javascript Question

d3 pictogram on a map: d3 use squares to represent counts

This might be a simple question, but I have a map in d3 and I'd like to represent event-counts as squares.

Here's an example png of what I'm going for:

rough sketch

They're not aligned perfectly in the picture, but let's say I have a JSON:

[
{city:'New York', count:3},
{city:'Washington, D.C.', count:1},
{city:'Austin', count:5},
{city:'Havana', count:8}
]


of counts that I'd like to represent as squares, preferably clustered in an orderly way.

I'm scratching my head on this — I think maybe a force-directed graph will do the trick? I've also seen this: http://bl.ocks.org/XavierGimenez/8070956 and this: http://bl.ocks.org/mbostock/4063269 that might get me close.

For context and set-up (I don't need help making the map, but just to share), here's the repo I'm using for the project: https://github.com/alex2awesome/custom-map, which shows the old way I was representing counts (by radius of a circle centered on each city).

Answer

does someone at least know what this might be called?

The technical name of this in dataviz is pictogram.

Here is a general code for plotting the rectangles, you'll have to change some parts according to your needs. The most important part is the calculation for the rectangles x and y position, using the modulo operator.

First, let's set the initial position and the size of each rectangle. You'll have to set this according to your coordinates.

var positionX = 5;
var positionY = 5;
var size = 5;

Then, let's set how many rectangles you want (this, in your code, will be d.count):

var count = 15;
var gridSize = Math.ceil(Math.sqrt(count));
var data = d3.range(count);

Based on the count, we set the gridSize (just a square root) and the data.

Now we plot the rectangles:

var rects = svg.selectAll(".rects")
    .data(data)
    .enter()
    .append("rect");

rects.attr("width", size)
    .attr("height", size)
    .attr("x", function(d,i){ return positionX + (i%gridSize)*(size*1.1)})
    .attr("y", function(d,i){ return positionY + (Math.floor((i/gridSize)%gridSize))*(size*1.1) })
    .attr("fill", "red");

Here is a working snippet, using 15 as count (4, 9, 16, 25 etc will give you a perfect square). Change count to see how it adapts:

var svg = d3.select("body")
	.append("svg")
	.attr("width", 50)
	.attr("height", 50);
	
var count = 15;
var size = 5;
var positionX = 5;
var positionY = 5;
var gridSize = Math.ceil(Math.sqrt(count));
var data = d3.range(count);

var rects = svg.selectAll(".rects")
	.data(data)
	.enter()
	.append("rect");
	
rects.attr("width", size)
	.attr("height", size)
	.attr("x", function(d,i){ return positionX + (i%gridSize)*(size*1.2)})
	.attr("y", function(d,i){ return positionY + (Math.floor((i/gridSize)%gridSize))*(size*1.2) })
	.attr("fill", "red");
<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.4.11/d3.min.js"></script>