geom_path() refuses to cross over the 0/360 line in coord_polar()

Question

I'm trying to plot the angle of an object (let's say it's a weather vane) over time. I want to plot it on a polar coordinate system and have the time points be connected by a path, showing how the angle evolves over time. I simply have a dataframe, with one column being the angle in degrees (numeric) and then the time step when the angle was recorded (integer).

But when I run the below code:

ggplot(df, aes(x = angle.from.ref, y = time.step)) +
  coord_polar() + 
  geom_path() + 
  geom_point() +
  scale_x_continuous(limits = c(0, 360), breaks = seq(0, 360, 45))

I get something that looks like this:

The path created by geom_path() refuses to cross the 0/360 degree line. If a value of 359 is followed by a value of 1, the path will not create a short link passing across the x=0/360 point. Instead, the path curves back ALL the way around the circle, arriving at x=1 from the other side.

I had hoped using coord_polar() would have solved this, but clearly not. Is there some way I can tell ggplot that the values 0 and 360 are adjacent/contiguous?

Answer 1

Ok my implementation is a bit hacky, but it might solve your problem. The idea is to simply implement a version of geom_point() that draws lines instead of points.

First, we'll need to build a ggproto object that inherits from GeomPoint and modify the way it draws panels. If you look at GeomPoint$draw_panel, you'll see that our function is virtually the same, but we're using polylineGrob() instead of pointsGrob().

GeomPolarPath <- ggproto(
  "GeomPolarPath", GeomPoint,
  draw_panel = function(data, panel_params, coord, na.rm = FALSE){
    coords <- coord$transform(data, panel_params)
    ggplot2:::ggname(
      "geom_polarpath",
      polylineGrob(coords$x, coords$y,
                   gp = grid::gpar(col = alpha(coords$colour, coords$alpha),
                                   fill = alpha(coords$fill, coords$alpha),
                                   fontsize = coords$size * .pt + coords$stroke * .stroke/2,
                                   lwd = coords$stroke * .stroke/2))
    )
  }
)

Now that we have that, we just need to write the usual function for geoms to accept this in layers. Again, this does the same thing as geom_point(), but passes GeomPolarPath instead of GeomPoints to the layer.

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

Finally, we can happily plot away all we want (blatantly stealing dww's example data):

ggplot(df, aes(x = angle, y = time.step)) +
  coord_polar() + 
  geom_polarpath() +
  geom_point() +
  scale_x_continuous(limits = c(0, 360), breaks = seq(0, 360, 45))

And here we go. I've only tested this for this particular plot, so I would expect some bugs and wierdness along the way. Potential downside is that it draws straight lines between points, so it doesn't curve along the angles. Good luck!

EDIT: You might need to load the grid package for this to work.

Answer 2

It may be more straightforward to bypass the crossing-over problem: interpolate at the 360/0 point, and plot each revolution as its own section. Here's how it can work:

library(dplyr)
library(ggplot2)

# sample data
n <- 100
df <- data.frame(
  angle.from.ref = seq(0, 800, length.out = n),
  time.step = seq(Sys.time(), by = "min", length.out = n)
)

df %>%
  interpolate.revolutions() %>%
  ggplot(aes(x = angle.from.ref, y = time.step, 
             group = revolution)) +
  geom_line(aes(color = factor(revolution)), size = 1) + # color added for illustration
  scale_x_continuous(limits = c(0, 360),
                     breaks = seq(0, 360, 45)) +
  coord_polar()

Code for interpolate.revolutions function:

interpolate.revolutions <- function(df, threshold = 360){
  # where df is a data frame with angle in the first column & radius in the second

  res <- df

  # add a label variable such that each span of 360 degrees belongs to
  # a different revolution
  res$revolution <- res[[1]] %/% threshold

  # keep only the angle values within [0, 360 degrees]
  res[[1]] <- res[[1]] %% threshold

  # if there are multiple revolutions (i.e. the path needs to cross the 360/0 threshold), 
  # calculate interpolated values & add them to the data frame
  if(n_distinct(res$revolution) > 1){        
    split.res <- split(res, res$revolution)
    res <- split.res[[1]]
    for(i in seq_along(split.res)[-1]){
      interp.res <- rbind(res[res[[2]] == max(res[[2]]), ],
                          split.res[[i]][split.res[[i]][[2]] == min(split.res[[i]][[2]]), ])
      interp.res[[2]] <- interp.res[[2]][[1]] + 
        (threshold - interp.res[[1]][1]) / 
        (threshold - interp.res[[1]][1] + interp.res[[1]][2]) *
        diff(interp.res[[2]])
      interp.res[[1]] <- c(threshold, 0)          
      res <- rbind(res, interp.res, split.res[[i]])
    }
  }
  return(res)
}

This approach can be applied to multiple lines in a plot as well. Just apply the function separately to each line:

# sample data for two lines, for different angle values taken at different time points
df2 <- data.frame(
  angle.from.ref = c(seq(0, 800, length.out = 0.75 * n),
                     seq(0, 1500, length.out = 0.25 * n)),
  time.step = c(seq(Sys.time(), by = "min", length.out = 0.75 * n),
                seq(Sys.time(), by = "min", length.out = 0.25 * n)),
  line = c(rep(1, 0.75*n), rep(2, 0.25*n))
)


df2 %>%
  tidyr::nest(-line) %>%
  mutate(data = purrr::map(data, interpolate.revolutions)) %>%
  tidyr::unnest() %>%

  ggplot(aes(x = angle.from.ref, y = time.step, 
             group = interaction(line, revolution),
             color = factor(line))) +
  geom_line(size = 1) +
  scale_x_continuous(limits = c(0, 360),
                     breaks = seq(0, 360, 45)) +
  coord_polar()