My Critique of Slope Graphs paper was recently rejected as a short article from The American Statistician. I’ve uploaded the new paper to SSRN with the suggested critiques and my responses to them (posted here).

I ended up bugging Nick Cox for some pre peer-review feedback and he actually agreed! (A positive externality of participating at the Cross Validated Q/A site.) The main outcome of Nick’s review was a considerably shorter paper. The reviews from TAS were pretty mild (and totally reasonable), but devoid of anything positive. The main damning aspect of the paper is that the reviewers (including Cox) just did not find the paper very interesting or well motivated.

My main motivation was the recent examples of slope graphs in the popular media, most of which are poor statistical graphics (and are much better suited as a scatterplot). The most obvious being Cairo’s book cover, which I thought in and of itself deserved a critique – but maybe I should not have been so surprised about a poor statistical graphic on the cover. This I will not argue is a rather weak motivation, but one I felt was warranted given the figures praising the use of slopegraphs in inappropriate situations.

In the future I may consider adding in more examples of slopegraphs besides the cover of Albert Cairo’s book. In my collection of examples I may pull out a few more examples from the popular media and popular data viz books (besides Cairo’s there are blog post examples from Ben Fry and Andy Kirk – haven’t read their books so I’m unsure if they are within them.) For a preview, pretty much all of the examples I consider bad except for Tufte’s original ones. Part of the reason I did not do this is that I wrote the paper as a short article for TAS — and I figured adding these examples would make it too long.

I really had no plans to submit it anywhere besides TAS, so this may sit as just a pre-print for now. Let me know if you think it may be within the scope of another journal that I may consider.


A critique of slopegraphs

I’ve recently posted a pre-print of an article, A critique of slopegraphs, on SSRN. In the paper I provide a critique of the use of slopegraphs and present alternative graphics to use in their place, using the slopegraph displayed on the cover of Albert Cairo’s The Functional Art as motivation – below is my rendering of that slopegraph.

Initially I wanted to write a blog post about the topic – but I decided to give all of the examples and full discussion I wanted it would be far too long. So I ended up writing a (not so short) paper. Below is the abstract, and I will try to summarize it in a few quick points (but obviously I encourage you to read the full paper!)

Slopegraphs are a popular form of graphic depicting change along two independent axes by means of a connecting line. The critique here lists several reasons why interpreting the slopes may be misleading and suggests alternative plots depending on the goals of the visualization. Guidelines as to appropriate situations to use slopegraphs are discussed.

So the three main points I want to make are:

  • The slope is not the main value of interest in a slopegraph. The slope is itself an arbitrary function of how far away the axes are placed from one another.
  • Slopegraphs are poor for judging correlation and seeing a functional relationship between the two values. Scatterplots or just graphing the change directly are often better choices.
  • Slopegraphs are difficult to judge when the variance between axes changes (which produce either diverging or converging slopes) and when the relationship is negative (which produces many crossings in the slopes).

I’ve catalogued a collection of articles, examples and other critiques of slopegraphs at this location. Much of what I say is redundant with critiques of slopegraphs already posted in other blogs on the internet.

I’m pretty sure my criminal justice colleagues will not be interested in the content of the paper, so I may need to cold email someone to review it for me before I send it off. So if you have comments or a critique of the paper I would love to hear it!