Vadim Khotilovich Vadim Khotilovich - 1 month ago 13
R Question

Hollow histogram or binning for geom_step

I would like to draw a hollow histogram that has no vertical bars drawn inside of it, but just an outline. I couldn't find any way to do it with

geom_histogram
. The
geom_step
+
stat_bin
combination seemed like it could do the job. However, the bins of
geom_step
+
stat_bin
are shifted by a half bin either to the right or to the left, depending on the step's
direction=
parameter value. It seems like it is doing its "steps" WRT bin centers. Is there any way to change this behavior so it would do the "steps" at bin edges?

Here's an illustration:

d <- data.frame(x=rnorm(1000))
qplot(x, data=d, geom="histogram",
breaks=seq(-4,4,by=.5), color=I("red"), fill = I("transparent")) +
geom_step(stat="bin", breaks=seq(-4,4,by=.5), color="black", direction="vh")


enter image description here

Answer

I propose making a new Geom like so:

library(ggplot2)
library(proto)

geom_stephist <- function(mapping = NULL, data = NULL, stat="bin", position="identity", ...) {
  GeomStepHist$new(mapping=mapping, data=data, stat=stat, position=position, ...)
}

GeomStepHist <- proto(ggplot2:::Geom, {
  objname <- "stephist"

  default_stat <- function(.) StatBin
  default_aes <- function(.) aes(colour="black", size=0.5, linetype=1, alpha = NA)

  reparameterise <- function(., df, params) {
    transform(df,
              ymin = pmin(y, 0), ymax = pmax(y, 0),
              xmin = x - width / 2, xmax = x + width / 2, width = NULL
    )
  }

  draw <- function(., data, scales, coordinates, ...) {
    data <- as.data.frame(data)[order(data$x), ]

    n <- nrow(data)
    i <- rep(1:n, each=2)
    newdata <- rbind(
      transform(data[1, ], x=xmin, y=0),
      transform(data[i, ], x=c(rbind(data$xmin, data$xmax))),
      transform(data[n, ], x=xmax, y=0)
    )
    rownames(newdata) <- NULL

    GeomPath$draw(newdata, scales, coordinates, ...)
  }
  guide_geom <- function(.) "path"
})

This also works for non-uniform breaks. To illustrate the usage:

d <- data.frame(x=runif(1000, -5, 5))
ggplot(d, aes(x)) +
  geom_histogram(breaks=seq(-4,4,by=.5), color="red", fill=NA) +
  geom_stephist(breaks=seq(-4,4,by=.5), color="black")

plot

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