Basile Simon (blog) Data, hackery, stories

A beeswarm layout in Observable -- on deadline

I recently had to complete a technical test for a role I'm interested in.

The task: from a story out of the Global Witness website, can you come up with an alternative visualisation? You've got two hours.

As there were no embed constraints, I figured there was no point in faffing with boilerplate code and scaffolds, so I went down the Observable route to put a chart together.

Here's what we need scales-wise: * On the y axis, the amount of the individual donations range from 0 to $10,000 -- we'll use a linear scale which domain is mapped to this extent; * On the x axis, we've got two categories: the donations that pass the NAACP criteria, and those that don't. We cue an ordinal scale which domain are these values; * A simple colour scale;


But as donation amounts overlap each other, so do our circles. Here they are, with a small increasing offset to show this banding effect:

Comes the force layout to the rescue, adding some simulated entropy to the x/y coordinates of our circles and allowing us to appreciate the difference in scale between the two columns.

const simulation = d3
  .forceSimulation(data)
  .force('x', d3.forceX(d => x(d.category)).strength(0.2))
  .force('y', d3.forceY(d => y(+d.donationUSD)).strength(1))
  .force('collide', d3.forceCollide(d => size(Math.sqrt(+d.donationUSD))))
  .alphaDecay(0)
  .alpha(0.3)
  .stop();

We apply three forces to our simulation: * One on the x axis, so our circles bucket into our two categories. We specify a weak .strength() so they spread out horizontally rather than vertically (and thus are more truthful to the precise donation amounts); * One on the y axis, where in this case the .strength() is far greater to squeeze them further, as seen above; * A collision force which repels the circles from each other, so as to spread them apart.

Here they are, showing the influence of x and y's .strength():

Did I say I had two hours to do this? I had to skimp on the annotations, which I've positioned manually and quite roughly as elements.


Read more: