geomtextpath
Create curved text in ggplot2
The existing text-based geom layers in ggplot2 (geom_text and geom_label) are ideal for the majority of plots, since typically textual annotations are short, straight and in line with the axes of the plot. However, there are some occasions when it is useful to have text follow a curved path. This may be to create or recreate a specific visual effect, or it may be to label a circular / polar plot in a more “natural” way.
Installation
You can install the development version of geomtextpath from GitHub with:
# install.packages("remotes")
remotes::install_github("AllanCameron/geomtextpath")
Examples
library(geomtextpath)
#> Loading required package: ggplot2
Plotting text along an arbitrary path
t <- seq(0, 5 * pi, length.out = 1000)
spiral <- data.frame(
x = rev(sin(t) * 1000:1),
y = rev(cos(t) * 1000:1),
s = seq(1, 10, length.out = 100),
text = paste(
"Like a circle in a spiral, like a wheel within a wheel,",
"never ending or beginning on an ever spinning reel"
)
)
ggplot(spiral, aes(x, y, label = text)) +
geom_textpath(size = 7, vjust = 2, linewidth = 0) +
coord_equal(xlim = c(-1500, 1500), ylim = c(-1500, 1500))
Produce labelled density lines
By default, the paths are broken to allow the names in-line.
ggplot(iris, aes(x = Sepal.Length, colour = Species)) +
geom_textpath(aes(label = Species), stat = "density",
size = 8, fontface = 2, hjust = 0.2)
If the vjust parameter moves the text above or below the line, the line is automatically filled in.
ggplot(iris, aes(x = Sepal.Length, colour = Species)) +
geom_textpath(aes(label = Species), stat = "density",
size = 8, fontface = 2, hjust = 0.2, vjust = -0.1)
Labelling groups of point along their trend line
ggplot(iris, aes(x = Sepal.Length, y = Petal.Length)) +
geom_point(alpha = 0.1) +
geom_textpath(aes(label = Species, colour = Species),
stat = "smooth", method = "loess", formula = y ~ x,
size = 8, linetype = 3, fontface = 2, linewidth = 3) +
scale_colour_manual(values = c("forestgreen", "deepskyblue4", "tomato4")) +
theme_bw()
Text paths in polar coordinates
Straight text paths in Cartesian coordinates become curved in polar coordinates.
df <- data.frame(x = 1:1000, y = 1, text = "This is a perfectly flat label")
p <- ggplot(df, aes(x, y, label = text)) +
geom_textpath(size = 6)
p
p + coord_polar(start = pi)
Limitations
There are limitations inherent in the plotting of text elements in ggplot due to the way that the underlying grid graphics handles text. A text string is dealt with as a zero-width object, and therefore the rotation and spacing of the letters making up the string can only be dealt with by treating each letter separately. Inevitably, this means that curved text paths have to be calculated based on the size and aspect ratio of the plotting device. Resizing the device after drawing a curved text path will therefore cause artefacts of spacing and rotation in the text.
It is important to realise that the letters are only rotated, and do not undergo any change in shape. Thus, for example, large text appearing on convex curves will not be deformed so that individual letters are narrower at the bottom and wider at the top. Doing so would require reinterpreting the letters as polygons.
Another issue is that we may wish to use a short curved label on a much longer path. Spacing the letters equally along the path would mean there is too much space between the letters for the label to remain legible. A single text string is therefore kept “together” according to the point size of the text in geom_textpath. This then leaves the problem of where on the path the text should be placed. This can be dealt with by the aesthetic mapping hjust, which allows the user to place the labels at the desired position along the path, including separate positions for each label.
A final point to note is that a path is usually a group-based geom (i.e. a path typically comprises x, y points from two columns over several rows of a data frame), whereas text labels can come from single rows in a data frame. This means that if we have a data frame with an x column, a y column and a grouping variable column, there can only be a single label for the group. Typically, this will be the grouping variable itself (see the examples, particularly those using the built-in iris data set.)
