Create specialized text geoms for ggplot2 and ggrepel

[stefano-meschiari]

Currently, using TeX() with ggplot2 to plot formatted text is markedly different from base graphics. With base graphics, calling

text(TeX(r"($\alpha + \beta$)"))

will plot the expression in the current plot.

In ggplot, the following will work:

ggplot(mtcars, aes(x=wt, y=mpg)) + 
geom_point() + 
xlab(TeX(r"(Car \textbf{weight})")) + 
ylab(TeX(r"(Car \textbf{fuel efficiency})")) # use the return value of TeX directly

but in the geoms like geom_text, geom_label, and annotate that take a label aesthetic, the input vector for the label aesthetic is expected to be of type character, rather than of type expression. In order to plot formatted text and formulas, the user is expected to pass a character representation of the plotmath expression and use the parameter parse=TRUE:

ggplot(mtcars, aes(x = wt, y = mpg)) + geom_point() +
annotate("text", x = 4, y = 25, label = "alpha+beta", parse=TRUE)

(note that in some cases, the geom will accept an expression, but will throw a warning).

The only way to use TeX in these situations is to make it return a character value and set parse to TRUE, e.g.:

ggplot(mtcars, aes(x = wt, y = mpg)) + geom_point() +
annotate("text", x = 4, y = 25, label = TeX(r"(\alpha+\beta)", parse=TRUE)

I propose to add the functions: geom_text_tex, geom_label_tex, and annotate_tex. These functions will forward to the underlying ggplot functions, and set the appropriate parameters for the user, such that:

ggplot(mtcars, aes(x = wt, y = mpg)) + geom_point() +
annotate_tex(x = 4, y = 25, label = r"(\alpha+\beta)")

will automatically parse TeX, turn it into an expression, and forward it to annotate with parse=TRUE.