[R] Replacing a plotting function in a package with a ggplot2 version

[Tom Hopper]

I’ve been working on a home-brewed version of the plot function in the package qcc that would be more compatible with grid graphics. I’d like to share this with the R community, in the hopes that this reference helps some of my fellow learners. The issues that I overcame were all documented elsewhere, but not always easy to find; hopefully this helps with the search engines.

I have written a replacement for the plot.qcc() function in the qcc package. Features that may be of interest:

* The new plot function generates graphs almost identical to the original base graphic versions, but using ggplot2, gtable and grid. Some of the differences in programming technique are documented (e.g. the need to pass data frames rather than individual vectors to ggplot). Several issues were overcome, including the search scope of aes() and the use of aes_string() versus aes(), especially when using ggplot within a function.

* The new function replaces plot.qcc() in the qcc namespace, so that after calling source() with my code, all calls to plot() with a qcc object draw the new ggplot2-based version. The code demonstrates how to unbind an existing function and bind a new function in its place within the package namespace using the following code:

unlockBinding(sym="plot.qcc", env=getNamespace("qcc"));
assignInNamespace(x="plot.qcc", value=plot.qcc, ns=asNamespace("qcc"), envir=getNamespace("qcc"));
assign("plot.qcc", plot.qcc, envir=getNamespace("qcc"));
lockBinding(sym="plot.qcc", env=getNamespace("qcc"));

* Finally, the code shows how to annotate a plot outside the data panel, when the labels need to be aligned with the data (e.g. plot a reference line with geom_hline() and then annotate a label for the line to the right of the plot).

This implementation might not be the most elegant or bug-free, but it has passed my testing so far.

The code is available on Github at https://github.com/tomhopper/gcc_ggplot/

Best Regards,

Tom Hopper
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