A geom similar to geom_text but curves in polar co-ordinates

[AllanCameron]

Consider the following example where we can't use geom_textpath because we only want labels at a single point rather than following a path:

df <- data.frame(region = c("North America", "South America", 
                            "Australasia", "Europe"), 
                 values = c(500, 200, 350, 600))

p <- ggplot(df, aes(region, values, fill = region)) + 
  geom_col(width = 0.5) + 
  lims(y = c(0, 750)) +
  labs(x = "") +
  theme_bw() +
  theme(legend.position = "none",
        axis.text.x = element_blank(),
        axis.ticks.length.x = unit(0, "mm"))

p1 <- p + geom_text(aes(label = region), size = 5, vjust = -1) 

p1

But if we switch to polar co-ordinates, the labels are of course straight:

p1 + coord_polar()

With what we have in place so far, it should be straightforward to define a new geom that acts as a drop-in for geom_text but bends in polar co-ordinates, so for example:

p2 <- p + geomtextpath:::geom_textpolar(aes(label = region), size = 5, vjust = -1) 

p2

and

p2 + coord_polar()

The implementation is something like this:

geom_textpolar <- function(
  mapping = NULL, data = NULL, stat = "identity",
  position = "identity", na.rm = FALSE, show.legend = NA,
  inherit.aes = TRUE,  ...
)
{
  layer(geom = GeomTextPolar, mapping = mapping, data = data, stat = stat,
        position = position, show.legend = show.legend,
        inherit.aes = inherit.aes,
        params = list(
          na.rm     = na.rm,
          ...
        ))
}

GeomTextPolar <- ggproto("GeomLine", GeomTextpath,
    extra_params = c("na.rm", "orientation"),

    setup_data = function(data, params) {
      data$linewidth <- 0
      data <- data[rep(seq(nrow(data)), each = 100),]
      if(!is.numeric(data$x)) data$x <- as.numeric(as.factor(data$x))

      text_width <- diff(range(data$x))/5
      range_seq <- seq(-text_width, text_width, length.out = 100)
      data$x <- data$x + range_seq

      data
    }
)

The questions are

1. Is this something that people would use (I'm sure I have looked for something similar before)
2. Do you immediately foresee any problems
3. I'm not too happy with the name. Can you think of a better one?

[teunbrand]

I think this is pretty nice and might have some use cases, but I'm questioning whether this should be a geom or a position adjustment. I don't foresee any problems that don't happen in Cartesian coordinates, but I do wonder if this only has a use case in non-Cartesian coordinates.

Similar to how geom_path() can be reparametrised into [x, y, xend, yend] in geom_segment() and vice versa, I do feel that alternative parametrisations might be useful in general. We could design some parametrisation that has a use cases in Cartesian coordinates too.

[AllanCameron]

You're right - it does only do something useful in polar co-ordinates. I do think there is an occasional need for something like this though - see for example this SO question and this other one. It would be great if we could get this to work as just a position adjustment, but I can't immediately see how that would work, since it involves passing row-wise data rather than vertically grouped data.

The implementation above is just a reparameterisation of geom_textpath, similar to the way that geom_line or geom_segment are created, but based on your comments I should probably focus on getting the main geom_textpath to work properly first, so that we can have something simple that works well and that we can build on.

[teunbrand]

I agree that it is useful to have, but I'm wondering whether there is an opportunity here to kill two birds with one stone. I haven't fully fleshed out anything yet, but I was thinking along the following lines.

• We could make a geom_textsegment() for placing text on a line segment parametrised by [x, y, xend, yend].
• We could also make a geom_textspoke() for placing text on some line segment parametrised by [x, y, angle, (length?)].

I imagine in both cases that the line pieces become curves in polar coordinates and because the line pieces are straight in cartesian coordinates, we can extend the hjust() parameters to beyond the [0, 1] interval as these are easily extrapolated (in contrast to non-straight curves).

The case you presented above would then be a geom_textspoke() wherein the (default) angle is 0.

[teunbrand]

Come to think of it, we could probably also make a geom_textspline() based on our answers here: https://stackoverflow.com/q/68786700/11374827.

[AllanCameron]

This enhancement is being archived as it forms part of a later thread, where it is linked for reference.