How to center geom_spoke around their origin - r

Question

The examples clearly show that geom_spoke draws lines originating at (x, y) of length radius pointing in the direction specified by angle:
ggplot(df, aes(x, y)) + geom_point() + geom_spoke(aes(angle = angle), radius = 0.5)
What is the simplest reusable way to center the spokes around (x, y)?
I prefer not to modify my data for it (that would be less easily reusable) or do inline trigonometry (also less reusable). I don’t mind if “radius” becomes “diameter” in the solution.

Answer 1

I think the easiest way to do this is going through the whole process of creating a Position subclass like e.g. in position_nudge
position_center_spoke <- function() PositionCenterSpoke
PositionCenterSpoke <- ggplot2::ggproto('PositionCenterSpoke', ggplot2::Position,
compute_panel = function(self, data, params, scales) {
# xend/yend is computed by this point, so shortcut!
data$x <- 2*data$x - data$xend
data$y <- 2*data$y - data$yend
#data$x <- data$x - data$radius*cos(data$angle)
#data$y <- data$y - data$radius*sin(data$angle)
# After shifting, the spoke needs to have diameter length,
# but I’m not sure if the radius is still used anywhere.
data$radius <- 2*data$radius
# Now the scales would need to be retrained,
# But compute_panel doesn’t allow that and
# compute_layer “should not be overridden”
data
}
)
Now you can easily use it:
expand.grid(x = 1:10, y=1:10) %>%
mutate(angle = runif(100, 0, 2*pi), speed = runif(100, 0, sqrt(0.1 * x))) %>%
ggplot(aes(x, y)) +
geom_point() +
geom_spoke(aes(angle = angle, radius = speed), position = 'center_spoke')