Plotmath and expressions about geomtextpath HOT 2 CLOSED

I have started implementing this. My approach was to keep the labels flat but angled rather than trying to curve them. Expressions need to be handled differently from text, but I thought the best thing to do was to try to create the same output structure that measure_text produces. This can't be done exactly, since you can't easily fit multiple expressions in a data frame column. However, we can do it in a list, so that mostly the same accessors and subsetters still work.

When we implement the "dumb" textpath, we should probably make every expression "dumb", though the placement is good on smooth curves at present.

Current behaviour is:

library(geomtextpath)
#> Loading required package: ggplot2

lab <- expression(paste("y = ", frac(1, sigma*sqrt(2*pi)), " ",
                            plain(e)^{frac(-(x-mu)^2, 2*sigma^2)}))

df <- data.frame(x = seq(-2, 0, len = 100),
                 y = dnorm(seq(-2, 0, len = 100)),
                 z = as.character(lab))

p <- ggplot(df, aes(x, y, label = z))

p + geom_textpath(vjust = -0.2, hjust = 0.1, size = 8, parse = TRUE)

and

ggplot() + xlim(0, 1) + 
  stat_function(fun = \(x) x, label = "paste('y = ', x)",
                geom = "textpath", vjust = -0.2, parse = TRUE) +
  stat_function(fun = \(x) x^2, label = "paste('y = ', x^2)",
                geom = "textpath", vjust = -0.2, parse = TRUE) +
  stat_function(fun = \(x) x^3, label = "paste('y = ', x^3)",
                geom = "textpath", vjust = -0.2, parse = TRUE)

from geomtextpath.

The latest commit allows multiple parsed expressions and fixes some line spacing problems with the above implementation, so plotmath should have the same functionality as text.

from geomtextpath.