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Mosaicplots in the ggplot2 framework: ggmosaic
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.
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 asmosaic(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 incranvas
allows very powerful interactions with the chart, but the dependency on Qt makescranvas
very hard to install (besides a specific version of Qt with tricky paths, it also needs both theqtbase
and theqtpaint
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 theggplot2
package and not integrated with it as ageom
, which makes it impossible to use additionalggplot2
tools such as facetting and layering except in very special cases.
With version 2.0.0 of the ggplot2
package the handling of geoms was completely revised, which makes the handling of user defined geoms much more straightforward and compliant with the remainder of the ggplot2
framework. This extension is based on ggproto
, which operates at the interface between ggplot2
and the more general proto
package.
We envision that the student will be using this approach and together with the main functionality of the productplots
package (such as the calculation routines, formatting, divider handling, ...) create an interface that allows an integration
of mosaicplots as a geom.
Because mosaicplots benefit greatly from additional user driven interactivity, a second part of this project is the creation of a shiny app that allows users to specify and change aspects of a mosaicplot interactively.
The outcomes of the project are:
- R package for generalized version of mosaic plots implemented as a
geom
for theggplot2
package based onggproto
. The package has to be fully functional and must be documented. - A set of examples documenting the use and flexibility of
geom_mosaic
. - 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.
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.
Once you have a solution to the medium or/and the hard problem, please get in touch with Heike Hofmann [email protected] and/or Dianne Cook.
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 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 extending
ggplot2
write a function that implements a geom of your choice. Document the function using Roxygen, and include it into an R package.
Students, please post a link to your test results here.
https://github.com/cryptomanic/Mosaic
https://github.com/haleyjeppson/mosaic
https://github.com/Eugllena/mosaicPlot
https://github.com/ShravanKarthik94/Mosaic
https://github.com/iNeil77/Mosaic
- Friendly M., Mosaic displays for multi-way contingency tables, Journal of the American Statistical Association, vol. 89, no. 425, pp. 190-200, 1994.
- Hartigan J. A. and B. Kleiner, Mosaics for contingency tables, in Computer Science and Statistics: Proceedings of the 13th Symposium on the Interface, (Fairfax Station, VA), pp. 268-273, Interface Foundation of North America, Inc., 1981.
- Hofmann H., Constructing and reading mosaicplots, Computational Statistics and Data Analysis, vol. 43, no. 4, pp. 565-580, 2003.
- Hummel J., Linked bar charts: Analysing categorical data graphically., Journal of Computational Statistics, vol. 11, pp. 23-33, 1996.
- Wickham H., ggplot2: Elegant graphics for data analysis. useR, Springer, July 2009.
- Wickham H., Hofmann H.: Product Plots IEEE Transactions on Visualization and Computer Graphics, InfoVis 2011, vol 17, no 12, pp. 2223-2230, 2011.
- Wilkinson L., The Grammar of Graphics. Statistics and Computing, Springer, 1999.