How to create a new geom for ggplot2

The new version of the ggplot2 package (v 2.0.0) will be available on CRAN in a few days. It has a very nice mechanism for adding new geoms and stats (more about it here).

Let’s create a new geom geom_christmas_tree() that will plot data with the use of christmas trees instead of points. The geom will support following aesthetics: size, fill, color, x and y.

The size will be translated into the number of segments of each single tree. Width of a tree is fixed and height is proportional to the size.

Let’s start with an example.

# generate random positions, colors and sizes mm = data.frame(matrix(runif(1200),300,4)) # plot ggplot(mm, aes(X1, X2, size=X3, fill=X4)) + geom_christmas_tree() + theme_void() + scale_fill_gradient(low='green1', high='green4')

One can combine this geom with other geoms (like contours).

ggplot(iris, aes(x=Sepal.Length, y=Petal.Length, size=Petal.Length, fill=Species)) + stat_density_2d(aes(color=Species)) + geom_christmas_tree() + theme_void() + theme(legend.position="none")

And here we have the definition of the geom.

GeomChristmasTree = ggproto('GeomChristmasTree', Geom, required_aes = c('x', 'y'), default_aes = aes(shape = 19, colour = 'black', fill = 'green4', size = 3, linetype = 1, alpha = 1, fontsize = 1), draw_key = draw_key_polygon,


draw_panel = function(data, panel_scales, coord) {

coords = coord$transform(data, panel_scales)
# each tree is 4*branch + 4

if (length(coords$size) == 1) {

tsize = rep(pmax(1, round(coords$size)), length(coords$x))

theight = rep(pmax(0, round(coords$size)), length(coords$x))

} else {

tsize = pmax(1, round(coords$size))

theight = pmax(0, coords$size)

}
# the scale factor

r01x = diff(range(coords$x))/100

r01y = diff(range(coords$y))/100
# coords

longx = unlist(lapply(seq_along(coords$x), function(i) {

if (tsize[i] == 1) {

dx = -c(0.3, 0.3, 1.2, 0, -1.2, -0.3, -0.3)

} else {

dx = -c(0.3, 0.3, rep(c(1.2,0.3), tsize[i]-1), 1.2, 0, -1.2, rep(c(-0.3,-1.2), tsize[i]-1), -0.3, -0.3)

}

r01x*dx + coords$x[i]

}))

longy = unlist(lapply(seq_along(coords$y), function(i) {

if (tsize[i] == 1) {

dy = c(-0.5, 0, 0, theight[i], 0, 0, -0.5)

} else {

dy = c(-0.5, 0, 0, rep(1:(tsize[i]-1), each=2), theight[i], rep((tsize[i]-1):1, each=2), 0, 0, -0.5)

}

r01y*dy + coords$y[i]

}))

longid = unlist(sapply(seq_along(coords$y), function(i) {

rep(i, each=4*tsize[i]+3)

}))
grid::polygonGrob(

longx,

longy,

id = longid,

gp = grid::gpar(col = coords[,'colour'],

fill = coords[,'fill'],

fontsize = 10)

)

}

)

geom_christmas_tree = function(mapping = NULL, data = NULL, stat = 'identity', position = 'identity', na.rm = FALSE, show.legend = NA, inherit.aes = TRUE, ...) { layer( geom = GeomChristmasTree, mapping = mapping, data = data, stat = stat, position = position, show.legend = show.legend, inherit.aes = inherit.aes, params = list(na.rm = na.rm, ...) ) }