So, I really think this one is pretty cool. We spend much of our time in R making graphs with data, but what if you have a theory that you’d like to express graphically? Something like what I’ll call “economics-style” graphs, illustrating, for example, the Solow growth model, a production–possibility frontier, or an indifference curve?

Well, rest assured that R can produce those, too, and it’s made simple by the bezier() function from Hmisc (Hmisc does a lot of other interesting things, but this is what you got in today’s Advent CalendaR slot). Bézier curves are a workhorse of vector graphics, and if you’re not familiar with them, I encourage you to become so, with this beautiful interactive demo and with this more detailed interactive demo.

The Gist shows you how to use Bézier curves to replicate Wikipedia’s Supply-and-Demand graph, and is pretty heavily commented, but I’ll add a few notes:

• Generating a Bézier curve with pre-specified x and y vectors takes some trial-and-error. Fortunately, it is usually a fun puzzle and it’s very quick to test. Just think of each point as “pulling” the curve toward itself.
• The script defines a hacky little function called approxIntersection(), which is intended to let you input two (x, y) vectors and will output their approximate intersection. This probably doesn’t work well in a lot of cases, and I would be interested in hearing of anyone’s less hacky solutions.
• Earlier drafts of this code required a bit of ggplot2 theme-wrangling, but with the release of ggplot2 0.9.3, theme_classic now produces the exact look I was going for.