ggplot2:::contour_breaks makes one bin less than expected

[tjebo]

Likely related, yet not quite the same: #3824

Following up on this Stackoverflow thread: https://stackoverflow.com/questions/61817440/follow-up-to-stat-contour-2d-bins-interpretation?noredirect=1#comment109350645_61817440

A previous version of ggplot2 3.2.1 seemed to have worked as expected with the correct number of bins.

This does make one bin less than expected:

library(ggplot2)

set.seed(1)
n=100
foo <- data.frame(x=rnorm(n, 0, 1), y=rnorm(n, 0, 1))

ggplot() +
  stat_density_2d(data = foo, aes(x, y), bins = 5, color = "black") +
  geom_point(data = foo, aes(x = x, y = y)) 

^{Created on 2020-05-15 by the reprex package (v0.3.0)}

The following fix results in expected behaviour:
in ggplot2:::contour_breaks, changing bins-1 to "bins" :

contour_breaks2 <- 
  function (z_range, bins = NULL, binwidth = NULL, breaks = NULL) {
    if (!is.null(breaks)) {
      return(breaks)
    }
    if (is.null(bins) && is.null(binwidth)) {
      breaks <- pretty(z_range, 10)
    }
    if (!is.null(bins)) {
      binwidth <- diff(z_range)/(bins)
    }
    if (is.null(breaks)) {
      breaks <- fullseq(z_range, binwidth)
    }
    breaks
  }

StatContour$compute_group <- function (data, scales, bins = NULL, binwidth = NULL, breaks = NULL, 
                                       na.rm = FALSE) 
{
  z_range <- range(data$z, na.rm = TRUE, finite = TRUE)
  breaks <- contour_breaks(z_range, bins, binwidth, breaks)
  isolines <- ggplot2:::xyz_to_isolines(data, breaks)
  path_df <- ggplot2:::iso_to_path(isolines, data$group[1])
  path_df$level <- as.numeric(path_df$level)
  path_df$nlevel <- scales::rescale_max(path_df$level)
  path_df
}

ggplot() +
  stat_density_2d(data = foo, aes(x, y), bins = 5, color = "black") +
  geom_point(data = foo, aes(x = x, y = y)) 

Session info

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[clauswilke]

There are five bins in your example. In any case, there is a fix related to this issue in current master. #3976

library(ggplot2)

set.seed(1)
n=100
foo <- data.frame(x=rnorm(n, 0, 1), y=rnorm(n, 0, 1))

ggplot() +
  stat_density_2d(data = foo, aes(x, y), bins = 5, color = "black", size = 3) +
  geom_point(data = foo, aes(x = x, y = y), color = "red", size = .1) 

^{Created on 2020-05-15 by the reprex package (v0.3.0)}

[tjebo]

@clauswilke Do you mean this tiny dot in the middle was indeed supposed to be a bin?

[tjebo]

What I forgot to say (my apologies)- Many thanks for your very prompt reply.

Although already posted in the linked thread, I believe it is more appropriate to post this here.

I kind of struggle to see the expected number of bins. Here using the example from the linked issue.

library(ggplot2)

ggplot() + 
  geom_contour_filled(data = faithfuld, aes(waiting, eruptions, z = density), bins = 3, alpha = 0.5) +
  stat_density_2d(data = faithful, aes(waiting, eruptions), bins = 3, color = "black", size = 1)

^{Created on 2020-05-15 by the reprex package (v0.3.0)}

Session info

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[clauswilke]

There seems to be a problem where the breaks are chosen awkwardly for stat_density_2d(). The case above shows this, where one bin is just a single point, and the following reprex also shows this. The third bin is there but is not visible. Could you open a new issue about that? I think it's specific to 2d densities, and it doesn't arise for 2d contours.

library(ggplot2)

ggplot() + 
  geom_density_2d_filled(data = faithful, aes(waiting, eruptions), bins = 3, alpha = 0.5) +
  geom_density_2d(data = faithful, aes(waiting, eruptions), bins = 3, color = "black", size = 1)

^{Created on 2020-05-15 by the reprex package (v0.3.0)}

[tjebo]

cheers for looking into that. I will open a new issue as requested