The graph above shows a simple example of a difference chart used as an example in the book [D3 Tips and Tricks](https://leanpub.com/d3-t-and-t-v7). It is based on the difference chart [example produced by Mike Bostock](http://bl.ocks.org/mbostock/3894205). 

The graph shows the difference in the number of daily downloads between [R Programming for Data Science](https://leanpub.com/rprogramming) by Roger Peng and [The Elements of Data Analytic Style](https://leanpub.com/datastyle) by Jeff Leek