Trevor Alexander Trevor Alexander - 8 months ago 61
R Question

What does autoplot.microbenchmark actually plot?

According to the docs,

"Uses ggplot2 to produce a more legible graph of microbenchmark timings."

Cool! Let's try the example code:

tm <- microbenchmark(rchisq(100, 0),
rchisq(100, 1),
rchisq(100, 2),
rchisq(100, 3),
rchisq(100, 5), times=1000L)

microbenchmark plots

I don't see anything about the...squishy undulations in the documentation, but my best guess from this answer by the function creator is that this is like a smoothed series of boxplots of the time taken to run, with the upper and lower quartiles connected over the body of the shape. Maybe? These plots look too interesting not to find out what is going on here.

What is this a plot of?


The short answer is a violin plot:

It is a box plot with a rotated kernel density plot on each side.

The longer more interesting(?) answer. When you call the autoplot function, you are actually calling

## class(ts) is microbenchmark

We can then inspect the actual function call via

R> getS3method("autoplot", "microbenchmark")
function (object, ..., log = TRUE, y_max = 1.05 * max(object$time)) 
    y_min <- 0
    object$ntime <- convert_to_unit(object$time, "t")
    plt <- ggplot(object, ggplot2::aes_string(x = "expr", y = "ntime"))
 ## Another ~6 lines or so after this

The key line is + stat_ydensity(). Looking at ?stat_ydensity you come to the help page on violin plots.