For a long time, I've been thinking about how to design a data visualization library that would make it easier to compose charts from simple components. On the one hand, there are charting libraries like Google Charts, which offer a long list of pre-defined charts. On the other hand, there are libraries like D3.js, which let you construct any data visualization, but in a very low-level way. There is also Vega, based the idea of grammar of graphics, which is somewhere in between, but requires you to specify charts in a fairly complex language including a huge number of transformations that you need to write in JSON.
My final motivation for working on this was the You Draw It article series by New York Times, which uses interactive charts where the reader first has to make their own guess before seeing the actual data. I wanted to recreate this, but for bar charts, when working on visualizing government spending using The Gamma.
The code for this was somewhat hidden inside The Gamma, but last month, I finally extracted all the functionality into a new stand-alone library Compost.js with simple and clean source code on GitHub and an accompanying paper draft that describes it (PDF).
In this article, I will show how to use Compost.js to implement a "You Draw" bar chart inspired by the NYT article. When loaded, all bars show the average value. You have to drag the bars to positions that you believe represent the actual values. Once you do this, you can click "Show me how I did" and the chart will animate to show the actual data, revealing how good your guess was. Before looking at the code, you can have a look at the resulting interactive chart, showing the top 5 areas from the 2015 UK budget (in % of GDP):