Wanted: A Perfect Scatterplot (with Marginals)

June 11, 2015
By

(This article was first published on Win-Vector Blog » R, and kindly contributed to R-bloggers)

We saw this scatterplot with marginal densities the other day, in a blog post by Thomas Wiecki:

NewImage

The graph was produced in Python, using the seaborn package. Seaborn calls it a “jointplot;” it’s called a “scatterhist” in Matlab, apparently. The seaborn version also shows the strength of the linear relationship between the x and y variables. Nice.

I like this plot a lot, but we’re mostly an R shop here at Win-Vector. So we asked: can we make this plot in ggplot2? Natively, ggplot2 can add rugs to a scatterplot, but doesn’t immediately offer marginals, as above.

However, you can use Dean Attali’s ggExtra package. Here’s an example using the same data as the seaborn jointplot above; you can download the dataset here.

library(ggplot2)
library(ggExtra)
frm = read.csv("tips.csv")

plot_center = ggplot(frm, aes(x=total_bill,y=tip)) + 
  geom_point() +
  geom_smooth(method="lm")

# default: type="density"
ggMarginal(plot_center, type="histogram")

I didn’t bother to add the internal annotation for the goodness of the linear fit, though I could.

PltggMarginal

The ggMarginal() function goes to heroic effort to line up the coordinate axes of all the graphs, and is probably the best way to do a scatterplot-plus-marginals in ggplot (you can also do it in base graphics, of course). Still, we were curious how close we could get to the seaborn version: marginal density and histograms together, along with annotations. Below is our version of the graph; we report the linear fit’s R-squared, rather than the Pearson correlation.

# our own (very beta) plot package: details later
library(WVPlots)
frm = read.csv("tips.csv")

ScatterHist(frm, "total_bill", "tip",
            smoothmethod="lm",
            annot_size=3,
            title="Tips vs. Total Bill")

PlotPkg

You can see that (at the moment) we’ve resorted to padding the axis labels with underbars to force the x-coordinates of the top marginal plot and the scatterplot to align; white space gets trimmed. This is profoundly unsatisfying, and less robust than the ggMarginal version. If you’re curious, the code is here. It relies on some functions in the file sharedFunctions.R in the same repository. Our more general version will do either a linear or lowess/spline smooth, and you can also adjust the histogram and density plot parameters.

Thanks to Slawa Rokicki’s excellent ggplot2: Cheatsheet for Visualizing Distributions for our basic approach. Check out the graph at the bottom of her post — and while you’re at it, check out the rest of her blog too.

To leave a comment for the author, please follow the link and comment on their blog: Win-Vector Blog » R.

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