Mosaicplots in the ggplot2 framework: ggmosaic

Background

Categorical variables are omni-present in today's data, and while there has been a lot of development for visualizations of categorical variables in recent years, graphical methods for categorical data and mixtures of qualitative and quantitative data are not well developed in comparison with what is available for numeric variables. One possibility of visualising multidimensional data are Mosaic plots proposed by Hartigan and Kleiner (1981). Enhanced with interactive features such as querying, re-ordering of variables and variable categories, and grouping of quantitative variables these plots become a very powerful and easy to use tool for analysing and understanding multivariate categorical data. Mosaic plots and, in particular, the one-dimensional spine plots are missing from ggplot2. While the productplots package is an implementation using ggplot2 graphics, it does not support the full functionality of ggplot2, such as e.g. facetting (for a variable not included in the prodplot) or additional layers (to show e.g. the 'density' of points within each category). Within ggplot2 using the position='fill' option in barcharts comes closest to showing a conditional feature allocation. This is no longer supported in qplot histogram. With the new ggplot2 version 2.0.0 (or, shortly 2.1.0) the way that geoms are support has been completely overhauled, and makes extensions much easier to write. We are proposing to add a mosaic geom to ggplot2 that allows to make use of the full functionality of ggplot2.

Related work

Mosaic plots have been implemented in a variety of packages: mosaicplot() is one of the base graphics in the stats package, mosaic() is part of the vcd package. Also part of the vcd package is strucplot(), providing an extension to mosaic(). qmosaic() is an interactive implementation of mosaic plots as part of the cranvas package, and the productplots package is an implementation based on the ggplot2 framework. Why do we need another implementation? I don't want to downtalk any of the existing solutions, but there are some unresolved issues in all of them, e.g.

• default spacing and labels are not quite right in the mosaicplot() implementation. From a data visualization point of view, it makes sense to make the best use of the space available, and the (default) spacing choices are not doing that.
• In the vcd implementation, there are some unintuitive ways, the formula gets resolved, e.g. mosaic(Improved ~ Treatment | Sex, data = Arthritis, zero_size = 0, highlighting_direction = "right") gives the same result as mosaic(Improved ~ Treatment + Sex, data = Arthritis, zero_size = 0, highlighting_direction = "right"). Statistically, not the same things are shown, and the chart should reflect that.
• the qmosaic implementation in cranvas allows very powerful interactions with the chart, but the dependency on Qt makes cranvas very hard to install (besides a specific version of Qt with tricky paths, it also needs both the qtbase and the qtpaint package to work)
• the implementation of the productplots package comes the closest to the envisioned result of this project. However, prodplot is functionality on top of the ggplot2 package and not integrated with it as a geom, which makes it impossible to use additional ggplot2 tools such as facetting and layering except in very special cases.

Details of your coding project

The outcomes of the project are:

• R package for generalized version of mosaic plots implemented as a geom for the ggplot2 package based on ggproto. The package has to be fully functional and must be documented.
• A set of examples documenting the use and the flexibility of the mosaic geom.
• A shiny app highlighting the mosaicplot functionality interactively, to allow users to specify parameters and see the impact immediately to allow them to familiarize to the more abstract concepts of mosaicplots.

Expected impact

I don't want to dissuade anybody from using their package of choice when drawing mosaic plots. Much rather do we want to reach the wider community of ggplot2 users to draw mosaic plots.

Mentors, please explain how this project will produce a useful package for the R community.

Mentors

Once you have a solution to the medium or/and the hard problem, please get in touch with [[https://github.com/heike ][Heike Hofmann]] hofmann@iastate.edu and/or [[https://github.com/dicook ][Dianne Cook]].

Tests

Several tests that potential students can do to demonstrate their capabilities for this particular project. Please modify the suggestions below to make them specific for your project.

• Easy: Install the [[https://github.com/hadley/productplots ][productplots package]] from github (you might have to install the devtools package first). Run one of the examples, put the chart in a knitr/Rmarkdown document and write a paragraph to explain the chart.
• Medium: write a shiny app that shows a mosaicplot (using prodplot) of a few variables and allows to interactively change at least one aspect of the mosaic.
• Hard: based on Hadley Wickham's introduction to [[http://docs.ggplot2.org/dev/vignettes/extending-ggplot2.html ][extending ggplot2]] write a function that implements a geom of your choice. Document the function using Roxygen, and include it into an R package.

Solutions of tests

Students, please post a link to your test results here.