How to make a custom ggplot2 geom with multiple geometries - r

Question

I've been reading the vignette on extending ggplot2, but I'm a bit stuck on how I can make a single geom that can add multiple geometries to the plot. Multiple geometries already exist in ggplot2 geoms, for example, we have things like geom_contour (multiple paths), and geom_boxplot (multiple paths and points). But I can't quite see how to extend those into new geoms.
Let's say I'm trying to make a geom_manythings that will draw two polygons and one point by computing on a single dataset. One polygon will be a convex hull for all the points, the second polygon will be a convex hull for a subset of the points, and the single point will represent the centre of the data. I want all of these to appear with a call to one geom, rather than three separate calls, as we see here:
# example data set
set.seed(9)
n <- 1000
x <- data.frame(x = rnorm(n),
y = rnorm(n))
# computations for the geometries
# chull for all the points
hull <- x[chull(x),]
# chull for all a subset of the points
subset_of_x <- x[x$x > 0 & x$y > 0 , ]
hull_of_subset <- subset_of_x[chull(subset_of_x), ]
# a point in the centre of the data
centre_point <- data.frame(x = mean(x$x), y = mean(x$y))
# plot
library(ggplot2)
ggplot(x, aes(x, y)) +
geom_point() +
geom_polygon(data = x[chull(x),], alpha = 0.1) +
geom_polygon(data = hull_of_subset, alpha = 0.3) +
geom_point(data = centre_point, colour = "green", size = 3)
I want to have a geom_manythings to replace the three geom_* in the code above.
In an attempt to make a custom geom, I started with code in geom_tufteboxplot and geom_boxplot as templates, along with the 'extending ggplot2' vignette:
library(ggplot2)
library(proto)
GeomManythings <- ggproto(
"GeomManythings",
GeomPolygon,
setup_data = function(self, data, params) {
data <- ggproto_parent(GeomPolygon, self)$setup_data(data, params)
data
},
draw_group = function(data, panel_scales, coord) {
n <- nrow(data)
if (n <= 2)
return(grid::nullGrob())
common <- data.frame(
colour = data$colour,
size = data$size,
linetype = data$linetype,
fill = alpha(data$fill, data$alpha),
group = data$group,
stringsAsFactors = FALSE
)
# custom bits...
# polygon hull for all points
hull <- data[chull(data), ]
hull_df <- data.frame(x = hull$x,
y = hull$y,
common,
stringsAsFactors = FALSE)
hull_grob <-
GeomPolygon$draw_panel(hull_df, panel_scales, coord)
# polygon hull for subset
subset_of_x <-
data[data$x > 0 & data$y > 0 ,]
hull_of_subset <-
subset_of_x[chull(subset_of_x),]
hull_of_subset_df <- data.frame(x = hull_of_subset$x,
y = hull_of_subset$y,
common,
stringsAsFactors = FALSE)
hull_of_subset_grob <-
GeomPolygon$draw_panel(hull_of_subset_df, panel_scales, coord)
# point for centre point
centre_point <-
data.frame(x = mean(coords$x),
y = coords(data$y),
common,
stringsAsFactors = FALSE)
centre_point_grob <-
GeomPoint$draw_panel(centre_point, panel_scales, coord)
# end of custom bits
ggname("geom_mypolygon",
grobTree(hull_grob,
hull_of_subset_grob,
centre_point_grob))
},
required_aes = c("x", "y"),
draw_key = draw_key_polygon,
default_aes = aes(
colour = "grey20",
fill = "grey20",
size = 0.5,
linetype = 1,
alpha = 1,
)
)
geom_manythings <-
function(mapping = NULL,
data = NULL,
stat = "identity",
position = "identity",
na.rm = FALSE,
show.legend = NA,
inherit.aes = TRUE,
...) {
layer(
geom = GeomManythings,
mapping = mapping,
data = data,
stat = stat,
position = position,
show.legend = show.legend,
inherit.aes = inherit.aes,
params = list(na.rm = na.rm, ...)
)
}
But clearly there are quite a few things not right in this geom, I must be missing some fundamental details...
ggplot(x, aes(x, y)) +
geom_point() +
geom_manythings()
How can I write this geom to get the desired result?

Answer 1

there are quite a few issues in your code, so I suggest you try with a simplified case first. In particular, the chull calculation was problematic. Try this,
library(ggplot2)
library(proto)
library(grid)
GeomManythings <- ggproto(
"GeomManythings",
Geom,
setup_data = function(self, data, params) {
data <- ggproto_parent(Geom, self)$setup_data(data, params)
data
},
draw_group = function(data, panel_scales, coord) {
n <- nrow(data)
if (n <= 2)
return(grid::nullGrob())
# polygon hull for all points
hull_df <- data[chull(data[,c("x", "y")]), ]
hull_grob <-
GeomPolygon$draw_panel(hull_df, panel_scales, coord)
# polygon hull for subset
subset_of_x <-
data[data$x > 0 & data$y > 0 ,]
hull_of_subset_df <-subset_of_x[chull(subset_of_x[,c("x", "y")]),]
hull_of_subset_df$fill <- "red" # testing
hull_of_subset_grob <- GeomPolygon$draw_panel(hull_of_subset_df, panel_scales, coord)
coords <- coord$transform(data, panel_scales)
pg <- pointsGrob(x=mean(coords$x), y=mean(coords$y),
default.units = "npc", gp=gpar(col="green", cex=3))
ggplot2:::ggname("geom_mypolygon",
grobTree(hull_grob,
hull_of_subset_grob, pg))
},
required_aes = c("x", "y"),
draw_key = draw_key_polygon,
default_aes = aes(
colour = "grey20",
fill = "grey50",
size = 0.5,
linetype = 1,
alpha = 0.5
)
)
geom_manythings <-
function(mapping = NULL,
data = NULL,
stat = "identity",
position = "identity",
na.rm = FALSE,
show.legend = NA,
inherit.aes = TRUE,
...) {
layer(
geom = GeomManythings,
mapping = mapping,
data = data,
stat = stat,
position = position,
show.legend = show.legend,
inherit.aes = inherit.aes,
params = list(na.rm = na.rm, ...)
)
}
set.seed(9)
n <- 20
d <- data.frame(x = rnorm(n),
y = rnorm(n))
ggplot(d, aes(x, y)) +
geom_manythings()+
geom_point()
(disclaimer: I haven't tried to write a geom in 5 years, so I don't know how it works nowadays)