Overlay geom_points(aes(shape)) on geom_boxplot?

Question

I am just trying to plot a ggplot figure of overlaying a points plot on a boxplot. I got very strange result and hope someone can tell me why and how to fix it. Overlay geom_points() on geom_boxplot(fill=group)? here is a similar question. But the key problem in mine is the shape.

Here goes an example:

library(ggplot2)
library(dplyr)

head(mtcars)

data = data.frame(
  x = factor(mtcars$vs),
  y = mtcars$wt,
  fill = factor(mtcars$am)
) %>% 
  dplyr::arrange(x, fill) %>% 
  dplyr::mutate(shape = rep(letters[1:4], 8))

set.seed(1)
ggplot(data, aes(x, y, fill = fill)) +
  geom_boxplot() +
  geom_point(position=position_jitterdodge())

I can get a plot:

Then I add shape mapping. You can see that all points changed completely. What I want is a same plot like above with only the point's shapes changed. i.e, the location of points should not change. I don't know why after adding shape mapping, the points are improperly assigned to the box group.

set.seed(1)
ggplot(data, aes(x, y, fill = fill)) +
  geom_boxplot() +
  geom_point(aes(shape = shape), position=position_jitterdodge())

Does the result with `geom_point(aes(shape = shape, group = fill), position=position_jitterdodge())` look like you expect? — Jon Spring, Dec 07 '21 at 05:44
@JonSpring Yeah! Could you post an answer so I can accept as a solution? — Shixiang Wang, Dec 07 '21 at 06:02

score 1 · Accepted Answer · answered Dec 07 '21 at 17:25

In this case we want the position jittering to be "aware" of the two fill values which are distinguished the the fill aesthetic. Since the shapes here don't have a fill aesthetic, the layer doesn't automatically separate the two fill values before applying the jitter. To make the layer "aware" of the fill values, we could use

geom_point(aes(shape = shape, group = fill), 
               position=position_jitterdodge())

where group = fill tells the layer to group the points based on that variable, which will then be reflected in the way the points are jittered.

Overlay geom_points(aes(shape)) on geom_boxplot?

1 Answers1