stat_density2d doesn't work nicely with geom_polygon

[JoFAM]

As per the title. If I use stat_density2d(aes(fill = stat(level)), geom = "polygon") the outer regions are completely warbled.

data(airquality)
airQ <- filter_all(airquality, ~!is.na(.))
ggplot(airQ, aes(x = Solar.R, y = Temp)) +
  geom_point() +
  stat_density2d(aes(fill = stat(level)),
                 geom = "polygon")

gives

[thomasp85]

The coming release will make you happy 🙂

[JoFAM]

@thomasp85 I hoped to be able to use this for the practical exercises tomorrow, but that's pushing it a bit probably? 😃

Definitely looking forward to it.

[thomasp85]

You can install the v3.3.0-rc branch and run with it

[clauswilke]

Looking into this now, I'm not sure this is fixed yet. We can draw filled contours from a raster grid of data, but we can't run the density estimation and then draw the filled contour, as far as I can tell.

library(tidyverse)

data(airquality)

airQ <- filter_all(airquality, ~!is.na(.))

ggplot(airQ, aes(x = Solar.R, y = Temp)) +
  stat_density_2d(aes(fill = after_stat(level)), geom = "polygon")

^{Created on 2020-03-01 by the reprex package (v0.3.0)}

Session info

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The reason is that StatDensity2d unconditionally calls StatContour. There is no way to make it call StatContourFilled.

ggplot2/R/stat-density-2d.r

Lines 79 to 81 in 8bc39f2

	if (contour) {
	StatContour$compute_panel(df, scales, bins, binwidth)
	} else {

[clauswilke]

This is how we'd have to do it if we calculate the density grid manually.

library(tidyverse)

data(airquality)

airQ <- filter_all(airquality, ~!is.na(.))

h <- c(MASS::bandwidth.nrd(airQ$Solar.R), MASS::bandwidth.nrd(airQ$Temp))
dens <- MASS::kde2d(
  airQ$Solar.R, airQ$Temp, h = h, n = 100,
  lims = c(range(airQ$Solar.R), range(airQ$Temp))
)
df <- expand.grid(x = dens$x, y = dens$y)
df$z <- as.vector(dens$z)

ggplot(df, aes(x, y, z = z, fill = after_stat(level))) +
  geom_contour_filled() +
  scale_fill_viridis_d()

^{Created on 2020-03-01 by the reprex package (v0.3.0)}

[clauswilke]

I just opened a PR (#3864):

library(tidyverse)

data(airquality)

airQ <- filter_all(airquality, ~!is.na(.))
ggplot(airQ, aes(x = Solar.R, y = Temp)) +
  geom_point() +
  stat_density2d(
    aes(fill = stat(level)),
    geom = "polygon",
    contour_type = "bands",
    alpha = 0.7
  )

^{Created on 2020-03-06 by the reprex package (v0.3.0)}