Rounded Corners in ggplot2 Graphics

February 11, 2011
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(This article was first published on sigmafield - R, and kindly contributed to R-bloggers)

Last night, while working on something else that I want to finish, I ended up indulging in a bit of "yak shaving". I wondered how easy it would be to generate graphics in ggplot2 with rounded corners. I don't think that there is any native support for this in the theming system. Therefore, I had to descend into the depths of R's grid graphics system. Luckily, I have read Paul Murrell's beyond excellent R Graphics book, so I was prepared for the venture.

Making my task even easier was the fact that there now exists the functionality in grid to generate rectangles with rounded corners. This is accomplished with the grid.roundrect and corresponding roundrectGrob functions.

So, here is my first attempt.

library(ggplot2)
p <- qplot(carat, price, data = diamonds[sample(1:nrow(diamonds), 100),]) +
           geom_smooth()

grob <- ggplotGrob(p)
plot.rrg <- roundrectGrob(gp = gpar(fill = "skyblue1", col = NA),
                          r = unit(0.06, "npc"))

grob <- removeGrob(grob, "plot.background.rect",
                   grep = TRUE)

grid.draw(gList(plot.rrg, grob))

ggplot2-rounded-corners.png

As you see, the objective was achieved, but not nearly as cleanly as I'd like. Essentially, I'm relying on a trick by removing the plot rectangle grob and creating a new one in a gList. This happens to work since the new round rectangle is drawn first. Ideally, I'd like to add the new rounded rectangle grob into the gTree object in the 'proper' place, i.e., where the original plot.background.rect rectangle was. I assume that is possible if I investigate further. It was not obvious to me how to do that though. Also, if I wanted the panel rectangle (grey colored in the figure) to be rounded, I believe I would have to find a way to replace that grob with a rounded rectangle at its original location.

So, that was a quick first attempt. The plot was saved using the png function with a transparent background. I will update this post if I find a cleaner solution. Please let me know if you are aware of how to add grobs in specific locations in a gTree, since that is what I am assuming I need to do.

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